Review Article Building Systems and
Swimming pool heating technology: A state-of-the-art review
Yantong Li1,2 (), Natasa Nord2, Gongsheng Huang1, Xin Li1
1. Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
2. Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway
A large amount of heat is needed to maintain the thermal comfort of both indoor and outdoor
swimming pools in cold seasons. This motivates the development of various heating technologies
aiming to reduce energy use, as well as operating and investment costs. Although their development
can be traced back to the 1960s, a comprehensive review of these technologies is lacking. Therefore,
this paper presents a comprehensive review of the development of heating technologies for
swimming pools. This review firstly introduces available heat transfer models that can be used to
calculate or predict heat loss and heat gain for swimming pools. Then, different passive and active
technologies are summarized. The active heating technologies used for indoor swimming pools
include solar collector, heat pump, waste heat recovery, geothermal energy, and congregation
technologies. The active heating technologies used for outdoor swimming pools include solar
collector, heat pump, PCM storage, geothermal energy, biomass heater, and waste heat recovery
technologies. A discussion is presented on the practical and possible heating techniques for swimming
pool applications. Finally, through the reviewed literature, future research opportunities are identified,
to guide researchers to investigate swimming pool heating systems with suitable and relevant
phase change material
Received: 15 January 2020
Revised: 11 May 2020
Accepted: 25 May 2020
© Author(s) 2020
Energy is a basic requirement of modern life (Xie et al.
2018; Li et al. 2020c) and also the precondition for the
development of industries in many respects, including
agriculture, transportation, telecommunication, digitalization,
etc. Since the industrial revolution in the 1760s, energy
demand has been rapidly increasing, due to economic and
social developments, together with sustained population
growth and rapid urbanization (Drissi Lamrhari and
Benhamou 2018; Du et al. 2017; Harkouss et al. 2018; Li
et al. 2019). The energy crisis is becoming more and more
serious, as fossil fuels are continuously utilized, leading to
environmental pollution problems (e.g. global warming and
ozone depletion) (Du et al. 2018, 2020). Hence, different
policies have been proposed to improve energy efficiency,
increase the use of renewable energy resources, and reduce
greenhouse gas emissions. The European Union has set goals
in its “2030 Climate and Energy Framework” for energy
efficiency to be increased by at least 32.5% from 2021 to
2030 (EU 2020), and for the share of renewable energy to
be increased to at least 32% from 2021 to 2030 (EU 2020).
In addition, by 2030, greenhouse gas emissions will be
cut by at least 40% from the 1990 levels (EU 2020). In
their “Renewable Energy Law”, the Chinese government has
proposed increasing the contribution of renewable energies
in electricity generation to 22.16% (Sakah et al. 2017).
Furthermore, in their 2020 project, the Korean government
has enacted the goal of increasing renewable energies’ con-
tribution to more than 20% of the total amount by 2030
(Kim et al. 2018).
Energy use in buildings comprises around 30% to 40%
of the total worldwide energy use (Berardi and Soudian 2018;
Hong et al. 2018; Uribe et al. 2018). Sports facilities are one
of the categories of buildings with the largest energy demand.
Compared with other sports facilities, swimming pools have
higher energy demand for pool water heating, ventilation,
spacing heating, and operation of circulation pumps (Kampel
et al. 2013). The annual energy use of a swimming pool facility
varies from 600 kWh/m2 to 6,000 kWh/m2, related to the
Li et al. / Building Simulation
List of symbols
Am pool area of each swimmer when the number of
swimmers is maximum (m2)
Ap surface area of the pool (m2)
As surface area of the conduction to ground (m2)
c a constant in Table 2 (W/(m2·Pa))
cc cloudiness factor (—)
cs specific heat of the soil (J/(kg·K))
cw specific heat of the water (J/(kg·K))
d a constant in Table 2 (W·s/(m3·Pa))
Ee water evaporation rate of the pool (kg/(s·m2))
Fa utilization factor of the pool (—)
Gs solar irradiation (W/m2)
He latent heat resulting from water evaporation (J/kg)
hcv convective heat transfer coefficient (W/(m2·K))
he evaporative heat transfer coefficient (W/(m2·Pa))
Ke a constant determined by density difference (—)
k thermal conductivity (W/(m·K))
ks thermal conductivity of the soil (W/(m·K))
Lc characteristic length of the pool for calculating
convective heat loss (m)
Ls characteristic length of the pool for calculating
conductive heat loss (m)
mrf mass flowrate of refilling fresh water (kg/s)
Ne number of swimmers (—)
Nu Nusselt number (—)
n a constant in Table 1 (—)
pa partial vapor pressure of the ambient air (Pa)
ps saturated vapor pressure at water surface (Pa)
Qcn heat loss from conduction (W)
Qcv heat loss from convection (W)
Qe heat loss from evaporation (W)
Qr heat loss from radiation (W)
Qrf heat loss from refilling water (W)
Qs dimensionless conduction heat rate (—)
Ra Rayleigh number (—)
RB Bowen ratio (—)
Ra relative humidity (%)
Sr specific humidity of room air (—)
Sw specific humidity of saturated air at water surface (—)
Ta temperature of ambient air (°C)
Tdew dew point temperature (°C)
Tp water temperature of the pool (°C)
Trf temperature of refilling fresh water (°C)
Ts temperature of the soil (°C)
Tsky sky temperature (°C)
Tsurf upper surface temperature of the ambient environment
Tw water surface temperature (°C)
t time (s)
Vp volume of the pool (m3)
wa wind speed parallel to the water surface (m/s)
we a coefficient in Table 1 (m/s)
x distance beneath the ground (m)
z a constant in Table 2 (—)
εw emissivity of the water (—)
ρr density of the room air (kg/m3)
ρs density of the soil (kg/m3)
ρsw density of the saturated air at water surface (kg/m3)
ρw density of the water (kg/m3)
σio a constant for distinguishing the ISPs and OSPs (—)
σs Stefan-Boltzmann constant (kW/(m2·K4))
γe a constant in Table 1 (—)
COP coefficient of performance (—)
CFD computational fluid dynamics
ISP indoor swimming pool
OSP outdoor swimming pool
PCM phase change material
volume and area of the pool, operation schedule, location,
weather, and types (Trianti-Stourna et al. 1998). The major
energy use of swimming pool facilities is for heating. The
heating purposes for indoor swimming pool (ISP) and
outdoor swimming pool (OSP) facilities differ.
People can swim at any time of the entire year in ISPs,
and swimming activities are unaffected by outdoor weather
conditions. The heat is required not only to maintain a
comfortable water temperature but also to ensure indoor
thermal comfort levels. Water evaporation will increase the
indoor humidity, resulting in an increase in ventilation
requirements (Rajagopalan and Jamei 2015). Thus, there is
a requirement for heat to be provided to warm the inducing
outdoor air. Kampel et al. (2013) collected the data of the
annual energy use of 41 Norwegian ISP facilities from 1998
to 2011. They concluded that the average annual energy use
of the Norwegian ISP facilities was around 3,991 kWh/m2.
They also reported that the climate was not the main factor
that affected the energy use of the swimming pool facilities,
but the water use of the pool had a strong relationship with
the energy use of the facilities (Kampel et al. 2016).
For OSPs, during the summer season, the high outdoor
temperature and solar irradiation can ensure a comfortable
pool temperature. However, during the cold season, especially
in subtropical climate regions, the OSP is closed due to the
undesirable weather (Li et al. 2020a). In some high-density
Li et al. / Building Simulation
cities like Hong Kong, the resource of space is very precious,
and thus the closure of the OSP will lead to the waste of the
space. Therefore, to increase the available opening times
for OSPs in the cold season, heating is required. Mousia
and Dimoudi (2015) evaluated the energy use of OSPs in
Greece, based on the data collected from questionnaires,
surveys, and the General Secretary of Sports. They found
that most staff used conventional heating technologies (e.g.
oil or gas boilers) to satisfy the OSPs’ heating demands. The
results of their data analysis showed that the average annual
energy use of an OSP was 2,456.16 kWh/m2, when a thermal-
insulation cover was not adopted, and 1,827.45 kWh/m2,
when a thermal-insulation cover was adopted. It could be
concluded that the energy use was high when traditional
heating technologies were adopted to supply heat for the
swimming pool facilities. Therefore, it is necessary to adopt
advanced heating technologies for swimming pool facilities.
However, to the best of the authors’ knowledge, a com-
prehensive review of suitable and advanced heating technologies
adopted for swimming pool facilities is lacking. There is an
urgent necessity to fill the knowledge gaps in developing
heat transfer models for swimming pools and the design
and operation of swimming pool heating systems.
This paper therefore presents an overview of advanced
heating technologies for swimming pools. Firstly, the
mathematical models, which were developed to describe the
heat transfer processes in swimming pools, including heat loss
from evaporative, convective, conductive, radiative, refilling
water, and heat gained from the sun are summarized. A
variety of empirical equations and approaches to describe
the main components of the swimming pool model are
presented. Secondly, the technologies utilized in swimming
pool heating systems are classified into passive and active
technologies. The active technologies are classified into ISP
and OSP heating technologies. ISP heating technologies
include solar collector, heat pump, waste heat recovery, and
geothermal energy technologies. OSP heating technologies
include solar collector, heat pump, PCM storage, geothermal
heat storage, biomass heater, and waste heat recovery
technologies. A discussion is presented on the practical and
possible heating techniques for both ISPs and OSPs. Finally,
future research opportunities are proposed, to guide scholars
to develop suitable and relevant swimming pool heating
The rest of the paper is organized as follows. The heat
transfer model of the swimming pool is described in
Section 2. Passive swimming pool heating technologies
are presented in Section 3, while Section 4 shows active
swimming pool heating technologies. Section 5 gives the
practical and possible heating techniques for swimming pool
applications, and Section 6 presents the concluding remarks
and possible future research opportunities.
2 Heat transfer model of the swimming pool
A swimming pool heat transfer model is the basic requirement
for investigating the performance of swimming pool heating
systems. It is proposed to describe the water variation of the
pool with the total heat flux of the pool, which is composed
of heat obtained from the sun and heat loss from evaporation,
convection, conduction, radiation, and refilling water. The
mathematical formula of this model has been presented in
the studies of Ruiz and Martínez (2010) and Woolley et al.
(2011), and it is expressed as:
ww p io s e cv cn r rf
ρcV σQ Q Q Q Q Q
⋅⋅⋅ - - - - -⋅= (1)
where ρw and cw are the density and specific heat of the
water, respectively; Vp and Tp are the volume and water
temperature of the pool, respectively; σio is a constant for
distinguishing ISPs and OSPs. For ISPs, σio is 0, while for
OSPs, σio is 1. Qs is the heat obtained from the sun; Qe, Qcv,
Qcn, Qr, and Qrf are the heat loss from the evaporation,
convection, conduction, radiation, and refilling water,
respectively; and t represents the time. Each of the above-
mentioned heat gain and loss items are described below in
2.1 Evaporative heat loss
The evaporative heat loss (Qe) is caused by the conversion
of the liquid water of the pool to gaseous water. The
calculations of the Qe of ISPs and OSPs differ and thus are
explained separately as follows.
(a) Evaporative heat loss in ISPs
For ISPs, Qe is calculated according to the water evaporation
rate (Ee), expressed below:
where Ap represents the surface area of the pool; and He
represents the latent heat resulting from the water evaporation.
Since the Ee in an occupied pool differs from that in an
unoccupied pool, different empirical equations are used
to calculate Ee for occupied and unoccupied pools, as
summarized in Table 1, where the coefficient (we) is deter-
mined by the following equation (Hanssen and Mathisen
ea a aw
0.12 4 1ww R TT
where wa represents the wind speed parallel to the water
surface; Ra represents the relative humidity; Ta represents
Li et al. / Building Simulation
the ambient air temperature; and Tw represents the water
In Table 1, Δp represents the difference between saturated
vapor pressure at the water surface and partial vapor
pressure of the room air; and Fa represents the utilization
factor of the pool, calculated by the following equation (Shah
where Am represents the pool area of each swimmer when
the number of swimmers is maximum; Ne represents the
number of swimmers; Ke is a constant determined by the
difference between the room air density (ρr) and the saturated
air density at water surface (ρsw); Ke is 35 when (ρr −ρsw) >
0.02 and Ke is 40 when (ρr −ρsw) ≤ 0.02 (Shah 2002, 2003); Sw
and Sr represent the specific humidity of the saturated air at
the water surface and room air, respectively; and γe and n are
constants that differ in different studies (Shah 2003, 2014).
In addition to the empirical equations summarized in
Table 1, different studies have been conducted to investigate
the evaporation phenomenon in ISPs. Asdrubali (2009)
established a swimming pool scale model in a climatic
chamber. The water evaporation ratio in different environ-
mental conditions was obtained and used to develop a new
predication model. Lu et al. (2014) adopted neural networks
to forecast the water evaporation rate in a swimming pool
facility in Finland. Furthermore, a computational fluid
dynamics (CFD)-based approach was developed for calculating
the water evaporation rate in the swimming pool facility
(Blázquez et al. 2017, 2018). Water evaporation had a
significant effect on the thermal-moisture status of the ISP
facilities. An experimental study on the thermal-moisture
environment in an ISP facility in different air variables was
presented in the study of Ciuman and Lipska (2018). Limane
et al. (2017, 2018) utilized OpenFOAM to model the mass
and heat transfer process in an ISP facility. They concluded
that the swimmers had an important influence on the indoor
environment in the swimming pool facility.
(b) Evaporative heat loss in OSPs
For OSPs, the evaporative heat loss (Qe) is calculated adopting
a semi-empirical correlation that includes the vapor pressure
difference and evaporative heat transfer coefficient, shown
as the following equation (Ruiz and Martínez 2010):
where ps is the saturated vapor pressure at water surface; pa is
the partial vapor pressure of ambient air; and he represents
the evaporative heat transfer coefficient that is regarded as the
empirical function of wind speed, depicted by the following
where c, d, and z are the factors that are identified by different
scholars, summarized in Table 2 (Ruiz and Martínez 2010;
Buonomano et al. 2015).
2.2 Convective heat loss
The convective heat loss (Qcv) is caused by the heat transfer
resulting from the movement of the pool water and ambient
air. It is calculated according to the temperature difference
between the water surface and ambient air, expressed as the
cv cv p p a
where hcv is the convective heat transfer coefficient.
In the ISP model, hcv is calculated according to Newton’s
law of cooling, depicted as the following equation (Winterton
Table 2 Different sets of factors for the evaporative heat transfer
coefficient (Ruiz and Martínez 2010; Buonomano et al. 2015)
Auth ors c (W/(m2·Pa)) d (W·s/(m3·Pa)) z (—)
Rohwer (1931) 0.0850 0.0508 1
McMillan (1971) 0.0360 0.0250 1
Richter (1979) 0.0423 0.0565 0.5
Smith et al. (1994) 0.0638 0.0669 1
ISO TC 180 (1995) 0.0506 0.0669 1
ASHRAE (2003) 0.0890 0.0782 1
Table 1 Summary of empirical equations of the Ee
References Equations (unoccupied pools) Equations (occupied pools)
Hanssen and Mathisen 1990
5 1/3 0.06 0.06
310 e e
Shah 2003, 2014 (general forms ) ee
Shah 2002, 2003
()EKρ ρρ SS⋅-⋅-⋅=
7.9 10 Δ0.113 5.9 10Ep
Shah 2012 e0.00005 ΔEp=⋅
Li et al. / Building Simulation
where k is the thermal conductivity; Lc is the characteristic
length of the pool; and Nu is the Nusselt number, expressed
as the following equation (Bergman et al. 2011):
0.14Nu Ra=⋅ (9)
where Ra is the Rayleigh number.
In the OSP model, hcv is calculated by the following
2.8 3.0hw=+⋅ (Lam and Chan 2001) (10)
3.1 4.1hw=+⋅ (Ruiz and Martínez 2010) (11)
In addition, Woolley et al. (2011) developed a new method
that is based on the Bowen formulation (Bowen 1926) to
calculate the convective loss, which is expressed as:
cv B e
where RB is the Bowen ratio, which can be calculated by
considering the effect of ambient pressure on the evaporative
and convective heat transmission (Bowen 1926).
2.3 Conductive heat loss
The conductive heat loss (Qcn) mainly results from the
temperature difference between the water of the pool and
soil. Many studies have indicated that Qcn is so small in the
total heat loss of the pool that it can be neglected. However,
Govaer and Zami (1981) reported that, in some cases, Qcn
should be considered, for example when there is moist
soil or sub-surface water movement, which may lead to a
significant increase in conductive loss. They gave the soil
temperature profile for calculating the conductive loss by the
governing equation, shown as follows:
ss s 2
⋅⋅ = ¶
where ρs, cs, ks, and Ts represent the density, specific heat,
thermal conductivity, and temperature of the soil, respectively;
and x represents the distance beneath the ground.
In addition, based on the assumption of Ts being uniform
and unchanged, Qcn can be calculated by (Buonomano et al.
cn s s s p s
where Ls is the characteristic length of the pool; Qs is the
dimensionless conduction heat rate that can be calculated
using shape factors (Bergman et al. 2011); and As represents
the surface area of conduction to ground.
2.4 Radiative heat loss
The radiative heat loss (Qr) is caused by the heat transfer
between the pool water and the upper surface of the ambient
environment through long-wave radiation. It is expressed as:
273 273QAεσT T=+-+⋅⋅⋅ (15)
where εw represents the emissivity of the water; σs is the
Stefan-Boltzmann constant, which is 5.67 ×10−11 kW/(m2·K4);
and Tsur is the upper surface temperature of the ambient
environment. In the ISP model, Tsur is the indoor surround-
ing surface temperature. In the OSP model, Tsur is the sky
temperature (Tsky), which can be calculated by the correlations
in Table 3. Tdew is the dew point temperature; εs is the
emissivity of the sky; and cc is the cloudiness factor.
2.5 Refilling water heat loss
The refilling water heat loss (Qrf) is caused by the tem-
perature difference between the water of the pool and the
refilling fresh water. The fresh water is required to refill the
pool, since the pool water is lost through evaporation and
draining. The Qrf is expressed as (Buonomano et al. 2015):
rf w rf p rf
where Trf represents the temperature of the refilling fresh
water; and mrf represents the mass flowrate of the refilling
fresh water. In the study of Buonomano et al. (2015), Trf
was constant at 15 °C. Different examples are presented to
discuss the values of mrf. A swimming pool operator’s
manual showed that Ontario’s regulations stated that, for
each swimmer, daily, 20 L of fresh water should be refilled
into the pool, and the maximum refilling water volume should
be 15% of the pool’s volume (McKeown 2009). Italian
standard UNI 10637 stated that the daily refilling water
volume of a pool was 5% of the pool volume (Buonomano
et al. 2015). Further, in a book, titled Reform in School
Mathematics and Authentic Assessment (Romberg 1995), a
Table 3 Different correlations of the Tsky
Smith et al. 1994
sky a dew
( 273) 0.8 / 250 273TT T=+ ⋅ + -
Ruiz and Martínez
(Buonomano et al.
sky a s s c
( 273) 0. 28317TT ε εc=+ ⋅+ -⋅- ⋅
Woolley et al. 2011 0.25
sky a s
(273) 273TT ε=+ ⋅ -
Li et al. / Building Simulation
case study showed that the daily refilling water volume of a
pool was 3% of the pool volume.
2.6 Heat gained from the sun
For an ISP, the thermal energy from the sun is absorbed by
the buildings, and it will affect the temperature of the room
air. The heat transfer between the water of the pool and the
room air will be affected. However, the water of the pool
cannot be directly obtained from heat from the sun, and
thus the heat gained from the sun (Qs) is not considered in
the heat transfer model of the ISP. For an OSP, the thermal
energy from the sun can be directly absorbed by the pool.
The Qs is expressed by the following equation (Lam and
where Gs denotes the solar irradiation; and αs denotes the
solar absorptivity, which is assumed to be 0.85 in the studies
of Lam and Chan (2001) and Ruiz and Martínez (2010).
However, in the study of Woolley et al. (2011), an annual
average absorption coefficient calculated by the approach
proposed by Wu et al. (2009) was adopted.
The techniques reviewed in this paper were classified
into passive and active techniques for swimming pool
applications. This classification was proposed according
to the information presented on the website of Varming
Consulting Engineers Ltd. (Varming 2020). The difference
between the passive and active design applications is presented
as follows. The passive design applications use the “natural
force” (e.g. sunlight, wind, and gravity) to realize the goals of
heating, cooling, and ventilation, and thus no fuel or grid
power is utilized. For example, roof ponds can be utilized
as a kind of passive heating technique (Sharifi and Yamagata
2015). The active design applications use electricity and fuel,
such as solar collectors and wind turbines, to realize the goals
of heating, cooling, and ventilation.
3 Passive techniques for swimming pool heating
The most commonly used passive technique for swimming
pools is to use a thermal-insulation cover, which can
effectively prevent heat losses, especially regarding evaporation
loss. That is to say, a thermal-insulation cover can effectively
reduce the heating load of a swimming pool, without using
fuel or grid power, and thus it belongs in the passive heating
technique category. The suggestion to utilize swimming
pool covers was first proposed by Brooks (1955). In 1960,
Root (1960) proposed an inflated plastic cover applied to
a swimming pool. The cover was inflated and floated on
the pool’s surface when the pool was not in use and could
be deflated and rolled up by a pontoon for storage when
the pool was utilized. Czarnecki (1963) tested on-site the
performance of an inflated polyvinyl chloride cover, with a
thickness of 0.02 cm, used on an OSP in Melbourne,
Australia. The author concluded that the strength of the
tested cover was not satisfactory and suggested the use of a
cover with a thickness of 0.05 cm. Table 4 depicts the thermal
properties of the typical material used in thermal-insulation
Comparisons between the performance of opaque and
transparent covers have also been studied. Szeicz and
Mcmonagle (1983) compared the performance of opaque
and transparent covers and concluded that, during the
night, an opaque cover was more effective for maintaining
the temperature of swimming pools than a transparent
cover, because the opaque cover was able to cut down long-
wave radiation. However, during a sunny day, the use of a
transparent cover obtained more thermal energy from the
sun than the use of an opaque cover. During a cloudy day,
a transparent cover might be less effective than an opaque
cover. Furthermore, Francey et al. (1980) reported an on-site
study investigating the performance of an air-bubble cover
used on an OSP, in which the performance of opaque and
transparent covers was compared. They found that the
transparent cover was more effective in raising the water
temperature than the opaque cover, since it could absorb
more sunlight. Figure 1 shows the schematic diagrams
of (a) single-layer; (b) inflated; and (c) air-bubble thermal-
insulation cover. Inflated and air-bubble thermal-insulation
cover might have better thermal preservation performance
than single-layer thermal insulation cover, since the thermal
resistance of the entire cover might be improved when the
air is integrated into the cover.
Table 4 Thermal properties of typical materials used in thermal-insulation covers
Names Density (kg/m3) Thermal conductivity (W/(m·K)) Specific heat (kJ/(kg·K)) References
Polyvinyl chloride 1100–1450 0.13–0.28 1.0 MEPC 2020
High-density polyethylene 930–970 0.46–0.52 2.3 MEPC 2020; OMNEXUS 2020
Low-density polyethylene 910–940 0.33 2.3 MEPC 2020; OMNEXUS 2020
Unnamed plastic — 0.08 — Szeicz and McMonagle 1983
Li et al. / Building Simulation
Fig. 1 (a) Single-layer; (b) inflated; and (c) air-bubble thermal-
4 Active techniques for swimming pool heating
This section presents the active techniques for swimming
pool heating application, including conventional, indoor,
and outdoor swimming pool heating techniques. Conventional
swimming pool heating techniques are reviewed to reflect
their current problems, demonstrating the urgent requirement
for advanced heating techniques. The reviewed advanced
techniques are classified into ISP and OSP heating techniques,
due to the following differences. Firstly, ISPs operate all year
round, and thus ISP heating techniques should be designed
to satisfy their annual heating demand. OSPs’ active season
normally only runs from May to September in subtropical
climate regions. To extend the available season of OSPs, OSP
heating techniques are designed to satisfy their cold-season
heating demands. Secondly, ISP heating techniques are
used not only to provide heat for the water of pool but also
to satisfy the indoor heating and ventilation demand. OSPs’
heating techniques are only used to provide heat for the
water of the pool. Generally, the energy consumption of
ISPs is relatively higher than that of OSPs because of the
high sensible, latent, and ventilation loads in ISPs. Relevant
results indicate that the energy use of an ISP is nearly three
times greater than that of an OSP of the same size
(Trianti-Stourna et al. 1998). It should be noted that the
techniques presented in this section are from the currently
4.1 Conventional swimming pool heating techniques
Conventional swimming pool heating techniques include
electric and oil/gas heaters. Electric heaters are used to
converse the electricity energy to thermal energy, and then
offer the heat to the pool. Oil/gas heaters, also called oil/
gas-fired heaters, are used to burn oil or gas to offer the heat
to the pool. Figure 2 shows the schematic diagram of electric
and oil/gas heaters for swimming pool heating applications.
Compared with oil/gas heaters, electric heaters might be
easier to be used. Electric heaters can immediately provide
heat for the pool when the power is turned on. Oil/gas heaters
generally comprise the fan, burner, and heat exchanger.
Ambient air will be input into the burner by fan and mixed
with oil. The mixture will be ignited, and the heat will be
transfer to the pool by heat exchanger. Compared with other
advanced techniques, electric and oil/gas heaters have the
advantage of quiet operation, reliability, and low initial
investment. However, they have the disadvantage of low
coefficient of performance (COP). Hence, in current literature,
they were selected as conventional heating techniques, and
the performance of systems using them was compared with
that using proposed advanced techniques. The performance
of systems with conventional heating techniques was related
to the price of the electricity, oil, and gas. Detailed information
about the price of the electricity, oil, and gas was presented
as follows. In the study of Brambley and Wells (1983), gas
and oil heaters were used, and the price of gas and oil were
$4.75/GJ and $1.20/gal, respectively. Croy and Peuser (1994)
used gas and oil heaters, and the price of energy costs was
0.05DM/kWh to 0.1 DM/kWh. In the studies of Lam and
Chan (2001, 2003) and Chan and Lam (2003), electric and
gas heaters were used, and the average price of the electricity
and gas in Hong Kong were HK$0.8/kWh and HK$0.21/MJ,
respectively. Katsaprakakis (2015) used the diesel oil heater,
and the annual average price of oil in Greece was 1.35€/lt.
Fig. 2 Schematic diagram of (a) electric and (b) oil/gas heaters for swimming pool heating applications
Li et al. / Building Simulation
4.2 Indoor swimming pool heating techniques
ISP heating techniques are utilized to maintain a suitable
pool water temperature within a comfortable thermal range
and satisfy the indoor thermal comfort requirement. This
section presents the current heating techniques applied in
ISPs, including solar collector, heat pump, waste heat recovery,
and geothermal energy technologies. In the following, a
brief overview of the mentioned heat supply technologies is
4.2.1 Solar collector technology
As one of the popular renewable energy sources, solar
energy has the advantages of cleanness, non-pollution, and
reliability. Therefore, it has been applied in many fields,
such as building cooling and heating (Feng et al. 2017; Li and
Huang 2019; Li et al. 2020d). The solar collector is regarded
as one main application of solar energy, which has been
extensively utilized to supply heat for ISPs. Figure 3 shows
the schematic diagram of an ISP heating system with solar
collectors. The water tank is used to store heat from solar
collectors, and the stored heat will be released to the ISPs.
The “on/off” of the associated pumps of the solar collector can
be controlled according to the real-time solar irradiance or
the temperature difference between the inlet and outlet
water temperature of the solar collector. The “on/off” of other
pumps can be controlled by the water temperature of the
pool. If the water temperature is lower than the set values
(e. g. 27 °C), the pumps will be turned on; otherwise, they
will be turned off. The heat exchanger is used to improve the
quality of the water that will be input to the ISPs. It should
be noted that, in some studies, the solar collectors are directly
linked with the ISPs, and thus the water tank and heat
exchanger might not be needed in the system. Heat collected
from solar collectors is directly supplied to the ISPs,
contributing to the reduction in heat loss from the water
tank and heat exchanger. Singh et al. (1989) developed a
simple analytical model for an ISP heating system with solar
collectors. Parametric studies on the effect of the solar
collector area and heat removal factor on the system per-
formance indicated that the water temperature of an ISP
increased as these two factors increased (Singh et al. 1989).
It should be noted that heat removal was a factor that was
usually used to calculate the useful heat gain of solar
collectors (Abdel-Khalik 1976). Tiwari and Sharma (1991)
developed an analytical model for calculating the energy
efficiency of the ISP heating system. Govaer (1984) analyzed
the energy performance of an ISP heating system with solar
collectors, using a utilizability method, which was applied
to evaluate the utilized thermal energy that could be provided
by solar collectors. It was concluded that the utilizability
Fig. 3 ISP heating system with solar collectors
theory could be effectively adopted to analyze the energy
performance of the system. Regarding economic indicators
of solar technologies as a heat supply solution for swimming
pools, Brambley and Wells (1983) reported that solar collector
technology adopted in the ISP heating system in St. Louis
might not be economical in comparison with a conventional
system with low-price fuels (e.g. gas), leading to an
unacceptable payback period of nearly ten years. However,
this fact might be argued, considering the current high
requirements for the use of renewables.
The photovoltaic/thermal collector (PV/T) is an advanced
application, which not only absorbs heat from the sun, like
solar collectors, but also produces electricity by converting
the absorbed thermal energy. The use of PV/Ts in ISPs has
been presented by different scholars. Buonomano et al.
(2015) utilized PV/Ts to provide heat and electricity for an
ISP in Naples, Italy. The surface area and volume of the pool
were 600 m2 and 1260 m3. A daily analysis of the system
showed that the electricity efficiency of the system was 9%,
and the thermal efficiency of the system was related to solar
irradiance. Bazilian et al. (2001) presented the design concept
for using PV/Ts in ISPs with a maximum heating temperature
of 30 °C. The installation of the glazed and unglazed PV/Ts
was designed for ISPs. They reported that the payback
period of the glazed PV/T was short, due to the utilization
of low-temperature heat.
4.2.2 Heat pump technology
In an ISP, the evaporation of pool water not only reduces
the water temperature of the pool but also increases the
indoor air humidity. To ensure indoor thermal comfort,
ventilation is necessary (Panaras et al. 2018), which leads to
a high energy demand for heating the inducing outdoor air.
In this case, a heat pump that dehumidifies the air can be
used to reduce ventilation load and thus to decrease the
energy use in the ISP heating system (Lee and Kung 2008).
Figure 4 shows the schematic of problems caused by the
evaporation of pool water and the functions of heat pumps
in ISP heating system. Heat pumps can be used in different
ways in ISPs, including open absorption heat pumps, heat
pump dehumidifiers, and solar assisted heat pumps.
Li et al. / Building Simulation
(a) Open absorption heat pump
Figure 5 shows the schematic diagram of an ISP heating
system with the absorption heat pump. The system is mainly
composed of absorber, evaporator, condenser, expansion
valve, generator, pump, and ISP. The absorption medium and
refrigerant are used in different cycles. In the absorption
medium cycle, the thermal energy is input to the generator.
The refrigerant with high pressure is separated from the
mixture and it is input to the condenser. The expansion
valve will be used to reduce the pressure of the absorption
medium. The absorption medium with low pressor will be
input to the absorber and it will be mixed with the refrigerant
from the evaporator. In the refrigerant cycle, the expansion
valve will be used to reduce the pressure of refrigerant from
the condenser. The heat obtained from the condenser and
absorber will be offered to the ISP. Westerlund and Dahl
(1994) developed a heating system with an open absorption
heat pump for an ISP, to reduce the heat demand for
dehumidification in a heating system. The exhausted air
from the ISP facility was dehumidified by the absorber. The
Fig. 5 ISP heating system with absorption heat pump
dehumidified air returned to the swimming pool facility,
and thus the energy use for handling the outdoor fresh air
was reduced. It was concluded that the initial investment of
such an open absorption heat pump might be paid back in
four to five years, if it was applied to the ISPs in Sweden.
These results indicated that this application was economically
feasible in some countries.
Similarly, Lazzarin and Longo (1996) proposed an open-
cycled heat pump system using heat recovery technology to
provide heat for an ISP, in which chemical dehumidification
was adopted in the exhausted air. It was reported that the
proposed heating system was cheaper and simpler than –
but had nearly the same energy-saving potential as – a
compressor-driven heat pump system. Furthermore, Johansson
and Westerlund (2001) compared the performance of the
ISP heating system using a mechanical heat pump with
those of an open absorption system and an air heater system.
The authors concluded that the energy use of the system
with the compressor-driven heat pump and that of the open
absorption system were reduced by 14% and 20%, respect-
tively, in comparison with the air heater system. However,
they reported that the energy use of the system adopting
the compressor-driven heat pump and that of the open
absorption system would increase, if the temperature in
the building increased. Consequently, in that case, the air
heater system would be considered the primary selection to
provide heat for the ISP.
(b) Heat pump dehumidifier
Another heat pump technology applied in ISPs is to use a
heat pump dehumidifier. Figure 6 shows a schematic of an
ISP heating system with a heat pump dehumidifier, which
was developed by Sun et al. (2011). The heat pump
dehumidifier had two functions: (1) to recover the latent
heat resulting from indoor wet air; and (2) to provide heat
Fig. 4 Schematics of (a) problems caused by evaporation of the pool water; and (b) functions of heat pumps in ISP heating system
Li et al. / Building Simulation
for the indoor air and the swimming pool. The switch of
these two modes was realized by changing the “on/off” state
of the valves in the system. The main difference between
this system and the open absorption system was that indoor
air was dried by the evaporator in this system, while it was
dried by the absorber in the open absorption system. The
system’s performance was evaluated, and it was concluded
that the system had good energy and economic savings
potential in comparison with a conventional system (i.e.
the system combining a conventional dehumidifier with an
electric boiler). Besides, the payback period of the system
was approximately 1.1 years.
(c) Solar assisted heat pump
To enhance the energy-saving potential of heat pumps,
solar assisted heat pumps have been proposed (Amin and
Hawlader 2013; Mohanraj et al. 2018). A few researchers
have explored the energy-saving potential of the solar assisted
heat pump applied in ISPs. Figure 7 shows the schematic of
an ISP heating system using a solar-assisted heat pump
developed by Tagliafico et al. (2012). Two operating modes
were considered in this system. In the first operating mode,
the by-pass pipes were closed, and the outlet water of the
solar collector was supplied into the evaporator. Thus,
the evaporator of the heat pump operated at a higher
environmental temperature than a traditional heat pump.
This led to a higher coefficient of performance (COP). In
the second operating mode, the heat pump was turned off,
and the by-pass pipes were open. The outlet water of the
solar collector was directly supplied into the water tank, to
satisfy the heating requirement of the ISP. The opening and
closing of the by-pass pipes were realized by the switch of
the valves. When the inlet water temperature of the solar
collector was between 4 °C and 18 °C, the system would
Fig. 7 ISP heating system with solar-assisted heat pump (Tagliafico
et al. 2012; reprinted with permission ©2012 Elsevier)
operate in the first mode; and when it was higher than 18 °C,
the system would operate in the second mode. The energy-
saving potential of the proposed heating system was evaluated
and found to be a function of degree days. Chow et al. (2012)
and Bai et al. (2012) evaluated the economic performance
of a solar-assisted heat pump for providing heat for an ISP
in winter. In this system, the solar collectors could be used
to provide heat for the ISP by means of a water storage tank.
The control for the “on/off” of the associated pumps in this
loop was realized by the water difference between the outlet
water temperature of the collector and the lower end tem-
perature of the storage tank. If the temperature difference
was between 2 °C and 10 °C, the associated pumps would
be turned on; otherwise, they would be turned off. The heat
Fig. 6 ISP heating system with heat pump dehumidifier (Sun et al. 2011; reprinted with permission ©2011 Elsevier)
Li et al. / Building Simulation
pump was responsible for the space heating. It would also
be used for heating the water of the pool when the solar
irradiance was weak. The authors concluded that the COP of
the system could reach up to 4.5, and the initial investment
of the system could be paid back in less than five years.
4.2.3 Waste heat recovery technology
Waste heat recovery can effectively improve the energy and
economic performance of a system, and thus it has been
extensively used in many fields, such as in air pre-purification
systems (Tong et al. 2018) and vehicle engines (Lan et al.
2018). It has also gained application in ISP heating systems.
For example, Oró et al. (2018) used the waste heat produced
by a liquid-cooled data center to heat an ISP, shown in
Figure 8. The heat produced by the data center would be
offered to the ISP. Meanwhile, the cold water from the ISP
could be used to cool the data center. The heat exchange
between the data center and the ISP was realized by the
heat exchanger. If the inlet water temperature was higher
than the set value (e.g. 20 °C), the cooling system would be
turned on; otherwise, it would be turned off. If the water
temperature of the pool was lower than the set value (e.g.
27 °C), the boiler would be turned on; otherwise, it would
be turned off. The economic performance of the system
was evaluated, and it was found that the operating cost of
the system was reduced by 18%. An ISP heating system that
recovered the waste heat from a chiller in an ice rink was
proposed by Kuyumcu et al. (2016). The evaporator of the
chiller was used to cool the ice rink down, and the heat
from the condenser of the evaporator would be stored in
an underground storage tank. The heat stored in this tank
would be released to the evaporator of a heat pump. This
would provide a high-temperature environment for the
evaporator, leading to a high COP of the heat pump. The
condenser of the heat pump was used to offer heat for the
ISP. The authors performed a parametric study of the heating
system, based on an analytical model to identify the optimal
ice rink size. Finally, the authors reported that the optimal
ice rink size was 475 m2, when the proposed heating system
was used in an ISP with an area of 625 m2.
4.2.4 Geothermal energy technology
Geothermal energy is an environmentally friendly source
of renewable energy, which has the merits of stability and
high capacity (Hou et al. 2018). In addition, unlike other
renewable energies, such as solar and wind, geothermal
energy is not easily influenced by the variations in weather
and seasons (Moya et al. 2018). Therefore, this energy source
can be regarded as reliable for providing heat for ISPs. A
typical heating technique for using the geothermal energy
is ground-source heat pump. Figure 9 shows the schematic
diagram of an ISP heating system with a ground-source
heat pump. The system mainly comprises a ground heat
exchanger, heat pump, pump, heat exchanger, and ISP. In
summer seasons, the extra heat from the ISP will be stored
into the ground by the ground heat exchanger. In winter
seasons, the stored heat will be extracted from the ground
heat exchanger and offered to the ISP. The typical sub-
components of the heat pump include the condenser,
expansion valve, evaporator, and compressor. As shown in
Figure 7 and Figure 12, the refrigerant will be cycled among
these four components. In summer seasons, the condenser
will connect with the side of the ground heat exchanger,
and the evaporator will connect with the side of the ISP. In
winter seasons, the evaporator will connect with the side of
the ground heat exchanger, and the condenser will connect
with the side of the ISP. A geothermal plant to supply heat
for an ISP in Naples, Italy, was suggested by Barbato et al.
(2018). In that study, the results of the cost analysis indicated
that the proposed system was economically feasible, because
the payback period of the system was 15.4 years. However,
this period of 15.4 years might be too long, and thus necessary
maintenance should be conducted in a fixed period to
ensure the normal operation of the system. In addition, this
Fig. 8 ISP heating system with the use of the waste heat from a data center (Oró et al. 2018; reprinted with permission ©2018 Elsevier)
Li et al. / Building Simulation
Fig. 9 ISP heating system with ground-source heat pump
study just calculated the possible cost according to the heating
load of the ISPs. A detailed design and control of the system
might not be presented, and thus the calculated results might
just be related to an initial evaluation of the application of
geothermal energy technology in an ISP heating system.
4.2.5 Cogeneration technology
Cogeneration, also called “Combined heat and power”, is a
technology that utilizes multiple components to integrate
an advanced system for offering heat and electricity.
Cogeneration technology has been applied in ISPs to satisfy
the heating and electricity demands. Facão and Oliveira
(2006) proposed a cogeneration system, comprising solar
collector, boiler, evaporator, turbine, electric generator, and
condenser in Lisbon, Portugal. The schematic diagram of
the congregation system was shown in Figure 10. The solar
collector and boiler were used to offer a suitable operating
environment for the evaporator. The turbine and electric
generator were used to produce the electricity. The condenser
was used to provide heat for the ISP, and the heat was
offered to the water of the pool and the space in the ISP.
The economic performance of the system was found to be
considerable when the solar collector area was between
10 m2 and 20 m2. Delmastro et al. (2015) evaluated the
performance of a biomass cogeneration system connected
to the district heating network in Torino, Italy. The produced
heat and electricity from the system were offered to the ISP.
It was concluded that the share of the renewable energy
sources would be improved, and the use of fossil fuels
would be reduced, when the designed system was used. In
addition, Wang et al. (2019) analyzed the performance of a
Fig. 10 ISP heating system with cogeneration technology (Facão
and Oliveira 2006; reprinted with permission ©2006 Oxford
cogeneration system with PV/T used for an ISP in Bari,
Italy. The system was found to satisfy 38.2% of the electricity
demand, 27.3% of the space heating demand, and 53.8% of
the hot water demand.
4.3 Outdoor swimming pool heating techniques
OSP heating techniques are utilized to maintain a suitable
water temperature for a pool, within a comfortable thermal
range. This section presents the current heating techniques
applied in OSPs, including solar collector, heat pump, PCM
storage, geothermal energy, biomass heater, and waste heat
recovery technologies. In the following, a brief overview of
the mentioned heat supply technologies is presented.
4.3.1 Solar collector technology
Solar collectors have been widely used in OSP heating
systems, due to their considerable energy-saving potential.
A description of the use of solar collectors in OSP heating
systems refers to Section 4.2.1 and Figure 3. Different
investigations of the use of solar collector technology are
presented as follows. Yadav and Tiwari (1987) proposed the
concept of adopting solar collectors with heat exchangers
to provide heat for an OSP. The authors found that the
water temperature of the OSP increases with the increase
in water flowrate, solar collector area and heat exchanger
length (Yadav and Tiwari 1987). Further, an experimental
study of an OSP heating system with solar collectors in
Germany showed that this system was economically com-
petitive when compared with a conventional heating system
(Croy and Peuser 1994). The results showed that, in the
cold season, around 250 to 300 kWh/m2 solar energy could
be used to supply heat for OSPs. In India, a heating system
with solar collectors was designed by Dang (1986), to supply
heat for an OSP. The author discovered that the calculated
energy efficiency of the solar collectors could reach up to
53.3%. However, for American conditions in the 1990s, the
life-cycle analysis of adopting solar collectors to supply heat
for an OSP in a university indicated that this heating system
might be not be economically feasible over a period of ten
years (Alkhamis and Sherif 1992).
A few researchers have focused on the development of
energy models for OSP heating systems with solar collectors.
For example, Rakopoulos and Vazeos (1987) developed an
energy model for an OSP heating system with solar collectors.
Based on the measured data for five OSPs heated by solar
collectors in Switzerland, Molineaux et al. (1994a) adopted
multilinear regressions to identify the empirical parameters
in the heating system models. Furthermore, the same
authors reported that the mean daily efficiency of the solar
collectors in this system could reach up to 60%, if the system
operated in optimal conditions (Molineaux et al. 1994b).
Li et al. / Building Simulation
Haaf et al. (1994) developed a simplified model of an OSP
heating system with solar collectors, which had the merits
of short calculation time and good user-friendliness.
Many studies have also been presented, to enhance the
performance of OSP heating systems. The utilization of a
low flow rate circulation pump is an efficient measure to
enhance a system’s energy efficiency. The effect of reducing
the flow rate on the performance of a solar collector heating
system adopted in an OSP was investigated by Cunio and
Sproul (2012), who found that the solar collector’s efficiency
was only reduced by 10%–15% when the flow rate was
decreased by up to 75%, leading to a reduction in the system’s
energy use of more than 80%. In addition, a simulation study
on the effect of a low flow rate circulation pump on the
performance of a solar collector with an area of 20.5 m2 for
an OSP with an area of 36 m2 was presented in the study of
Zhao et al. (2018). They found that 0.016 kg/(s·m2) was the
optimal mass flow rate for each unit collector area, in which
an energy-saving ratio of 60% could be achieved. Adjusting
the set-point temperature of swimming pools is another
approach for enhancing the performance of OSP heating
systems. For example, Hahne and Kübler (1994), who
conducted a simulation study of an OSP heating system
with solar collectors, found that the energy efficiency of
the system could be increased when the set-point water
temperature of the swimming pool was reduced within a
reasonable range, such as from 24 °C to 22 °C.
In addition, PV/T plants that can covert the solar
energy into both electricity and thermal energy (Furukakoi
et al. 2018) can improve the performance of OSP heating
systems. The performance of an OSP heating system with
PV/T, shown in Figure 11, was assessed by Buonomano
et al. (2015). In this study, the authors discovered that the
improvement in the energy performance of the system was
remarkable when the PV/T was used. They also discovered
that, due to the high initial investment in the system,
necessary incentive policies were needed to promote the
use of the system. Yandri (2017) reported that the energy
efficiency of an OSP heating system with PV/T could be
effectively improved by integrating Joule heating, which
was an internal heating technology that could produce heat
through an electrical conductor. Further, Clot et al. (2017)
Fig. 11 OSP heating system with PV/T (Buonomano et al. 2015;
reprinted with permission ©2015 Elsevier)
proposed a concept of submerging the PV/T in the water of
the OSP. This concept solved the difficulty of finding enough
space to install the PV/T. A simulation, using the submerged
PV/T in an OSP with a surface area of 60 m2 and a volume
of 75 m3 was conducted in Rome, Italy. It was found that
nearly 10 MWh electricity could be produced when the
system was used.
4.3.2 Heat pump technology
Heat pumps have been applied to OSP heating systems, and
they are only used to heat the water of the pool. Figure 12
shows the schematic diagram of an OSP heating system with
air-source heat pump. The system mainly comprises fan,
evaporator, compressor, condenser, expansion valve, and
OSP. The fan will cycle the ambient air that exchange the
heat with the refrigerant in the evaporator. The compressor
will be used to increase the pressure of the refringent from
the evaporator. The refrigerant with high pressure will be
input into the condenser and exchange heat with the cold
water from the OSP. The hot water will be supplied into the
OSP. The refrigerant leaving the condenser will be input into
the expansion valve and the pressure of the refrigerant will
be reduced. A life-cycle cost analysis of using heat pumps to
supply heat for an OSP in South Africa was performed in
the study of Greyvenstein and Meyer (1991). The results
indicated that using heat pumps to provide heat for an OSP
was more economically beneficial than using solar collectors.
The reason might be that the capital cost of using solar
collectors was greater than that of using heat pumps. In the
studies of Lam and Chan (2001, 2003) and Chan and Lam
(2003), heat pumps were used to provide heat for an OSP
of a five-star hotel in Hong Kong. They concluded that the
energy cost for the heating system with heat pumps with a
COP of 3.5 could be reduced by HK$ 275,700 in a ten-year
life cycle, in comparison with that of a conventional system
such as electrical or gas heaters.
4.3.3 PCM storage technology
Energy storage technology allows energy to be stored during
Fig. 12 OSP heating system with air-source heat pump
Li et al. / Building Simulation
low demand or high renewable energy generation periods,
and to be discharged during high demand or low renewable
energy production periods (Li et al. 2015; Argyrou et al.
2018; Du et al. 2019). This fact enables energy storage
technology to be broadly applied in renewable energy
applications. Compared with other energy storage media,
phase change materials (PCMs) have the merits of high energy
storage density and nearly constant temperature during the
phase transition process (Xu et al. 2017, 2018; Aketouane
et al. 2018; Li et al. 2020b). Thus, they have been extensively
adopted in many fields, such as domestic hot water systems
(Gorzin et al. 2018), building cooling systems (Bourne and
Novoselac 2015; Panchabikesan et al. 2020), and battery
thermal management systems (Li et al. 2018a). Recently,
researchers have started to use PCM storage in OSP heating
systems. For example, Zsembinszki et al. (2012) compared
two methods of applying PCMs in OSPs: (1) to install PCM
panels in the swimming pool walls, and (2) to use PCM
storage tanks to supply heat for OSPs. They concluded that
the latter was better than the former, because the discharging
time of the stored heat could be well controlled. Furthermore,
Li et al. (2018b,c) proposed an OSP heating system where
air-source heat pumps were used, together with PCM storage
tanks, as shown in Figure 13. The “on/off” controllers would
be used to control the open and closed states of the air-
source heat pumps and their associated pumps, according
to the formulated time schedule and setting temperature
values. During the electric off-peak period, the air-source
heat pumps and their associated pumps would be first
opened to store heat in the PCM storage tank. When the
temperature of the storage tank reached the set value (e.g.
60 °C), they would be closed. Then, they would be opened
again to preheat the water of the pool at the formulated
moment. When the water of the pool reached the set value
(e.g. 28.5 °C), they would be closed. In addition, during
the open period of the OSP, the PI controller was used to
maintain the water of the pool within a comfortable thermal
Fig. 13 OSP heating system proposed in the study of Li et al.
(2020b; reprinted with permission ©2020 Elsevier)
range (e.g. around 28 °C). The function of the PCM storage
tank was to shift the electricity use from the on-peak to the
off-peak period and thereby brought considerable economic
benefits. The results indicated that the proposed heating
system was economically and technically feasible.
4.3.4 Geothermal energy technology
Geothermal energy has also gained application in OSPs. A
description of the use of the geothermal energy technology
in OSP heating systems refers to Section 4.2.4 and Figure 9.
For example, Somwanshi et al. (2013) proposed an OSP
heating system, which collected heat from the soil at a depth
of four meters. The simulation results of this system indicated
that the water temperature of the pool could be maintained
between 22 °C and 27 °C in four different climatic regions,
and thereby the proposed heating system was technically
feasible. However, the calculation results were based on heat
transfer models, and detailed design and control strategies
were not considered in this study. In addition, the proposed
system might be technically feasible, but it might not be
economically feasible. An economic analysis of the system,
based on the proposed suitable design and control strategies,
should be conducted.
4.3.5 Biomass heater technology
Biomass is a renewable energy technology that has the merits
of being approximately carbon neutral and abundant in
many sources (Bajwa et al. 2018). Figure 14 shows an OSP
heating system with biomass heater. The biomass (e.g.
animal and agriculture residue) is input into the burner.
The fan will be used to blow the air into the burner. The
mixture of biomass and air will be ignited. The heat resulted
from the burning will be transferred to the OSP by the heat
exchanger. Based on this technology, Katsaprakakis (2015)
proposed two OSP heating systems: (1) using biomass
heaters and (2) using a combination of biomass heaters and
solar collectors. In the first system, the required heat of the
OSP came totally from the biomass heater. In the second
system, the required heat of the OSP came from both solar
collectors and biomass heaters. If the temperature of the
solar collectors was 4 °C higher than the water temperature
of the pool, the associated heat pump of the solar collector
would be turned on; otherwise, it would be turned off. The
biomass heater would be opened when the solar collector
could not offer enough heat to the water of the OSP. The
economic performances of these two systems adopted in
OSPs in Greece were evaluated. The authors reported that
the operating costs of these proposed systems were
significantly reduced in comparison with the conventional
systems that adopted expensive diesel oil. In addition, the
payback period of these systems was less than five years.
Li et al. / Building Simulation
Fig. 14 OSP heating system with biomass heater
4.3.6 Waste heat recovery technology
Waste heat recovery techniques are also found to supply
heat for OSPs. A description of the use of waste heat recovery
technology in OSP heating systems refers to Section 4.2.3
and Figure 8. For example, Borge et al. (2011) showed that
an OSP could be used as a heat sink for air-conditioning
systems and thereby reduce the heat demand of OSPs
and simultaneously enhance the energy efficiency of air-
conditioning systems. Furthermore, Harrington and Modera
(2013) conducted a feasibility study on rejecting waste heat
from air-conditioning systems into an OSP in California.
This could not only offer heat for the OSP but also reduce
the cooling energy use and peak demand of the buildings.
They concluded that the energy use for single-family cooling
purposes could be reduced by 25%–30%, when the proposed
system was applied.
5 Discussion on practical and possible techniques
for swimming pool applications
Section 4 presented the swimming pool heating techniques
from the existing literature. However, some key techniques
that can be applied for swimming pool applications might
not be reported in the existing literature. Discussion of these
techniques is valuable, and thus the practical and possible
techniques for swimming pool applications are presented
in this section. The details of the discussion are shown
5.1 Practical techniques in engineering projects
The practical techniques in engineering projects for swimming
pool applications are very important and worthy of review.
However, in current literature, few practical engineering
projects for swimming pool heating applications have been
reported. More practical engineering projects should be
investigated and presented in the literature. Scholars should
conduct more investigations of swimming pool heating
applications from practical aspects. Based on the authors’
current information, a complicated heating system with a
heat pump has been applied in an ISP in Norway. The heat
pump is used to provide heat for the water of the pool and
to satisfy the heating and ventilation load for the buildings.
The heat pump also provides heat for showering and connects
with geothermal heat storage devices. In addition, currently
there are some successful companies that have produced
different equipment to offer heat for the swimming pool
facilities. For example, a company called Degaulle completed
a project for an Indonesia hotel, in which four heat pumps
were used to offer heat for the facilities; and a drug disinfection
system and six sand filters were used to play the role in
disinfecting and filtering, respectively (Degaulle 2020b).
This company also completed a project for a Vietnam hotel,
in which four heat pumps, a drug disinfection system and
eight sand filters were used (Degaulle 2020a). Further, a
project titled Holmen Indoor Swimming Pool, Asker has
been completed (Holmen 2020). This ISP heating system
comprised the PV panels with the surface area of 650 m2,
solar collectors with the surface area of 650 m2, 15 geothermal
wells, and five heat pumps. PV panels were used to provide
heat and electricity for the facility. The annual electricity
produced by PV panels were 73,000 kWh, which satisfied
around 12% of the annual electricity demand of the facility.
Solar collectors were used to offer hot water. Geothermal
wells were used to recover the waste heat from grey water.
The recovered heat would be offered to the facility by heat
pumps. Hence, it could be found that, in practical applications,
if multiple heating techniques were used together, they
might effectively enhance the performance of the system.
However, in current literature, most scholars mainly focused
on single heating techniques; therefore, more investigations
into multiple heating techniques should be conducted.
5.2 Possible techniques for swimming pool applications
Section 4 presented the ISP and OSP heating techniques
from the existing literature. It can be seen that the currently
investigated ISP heating techniques include solar collector,
heat pump, waste heat recovery, geothermal energy, and
cogeneration technologies; meanwhile, the currently
investigated OSP heating techniques include solar collector,
heat pump, PCM storage, geothermal energy, biomass heater,
and waste heat recovery technologies. The solar collector,
heat pump, waste heat recovery, and geothermal energy
technologies have been investigated and presented in the
literature for both ISP and OSP applications. Hence, many
technologies could be used in both ISP and OSP applications.
However, considering the difference between ISP and OSP
heating techniques described at the beginning of Section 4,
the design of common techniques should consider two key
issues: (1) the annual operating time of an ISP technique
is longer than that of an OSP technique; (2) heating load
including ventilation load should be considered in ISP
Li et al. / Building Simulation
techniques, while it is not considered in OSP techniques. In
fact, the PCM storage and biomass heaters presented in OSP
heating techniques can be used for ISPs, while the mentioned
design suggestions should be considered. In addition, the
cogeneration presented in ISP heating techniques can be
used for OSPs. It should be noted that fuel cells that can
effectively convert the chemical energy into electricity can
be used in cogeneration techniques for both ISPs and OSPs.
Currently, few studies have focused on investigating the
use of fuel cells in ISP and OSP heating systems. Based on
the above discussion, more studies should be conducted
regarding the use of PCM storage and biomass heater
techniques for ISP heating systems, using cogeneration
techniques for OSP heating systems and fuel cells for both
ISP and OSP heating systems.
6 Conclusions and future possible research
This review summarized the heat transfer model for swimming
pools, and passive and active technologies for swimming
pool heating systems. Different equations and approaches
for calculating the components in the heat transfer model
of the swimming pool, including heat loss from evaporative,
convective, radiative, conductive, refilling water and heat
gained from the sun, were summarized. The active heating
technologies for ISPs were classified as solar collector,
heat pump, waste heat recovery, geothermal energy, and
cogeneration technologies. The active heating technologies
for OSPs were classified as solar collector, heat pump, PCM
storage, geothermal energy, biomass, and waste heat recovery
technologies. A discussion was presented on the practical and
possible heating techniques for swimming pool applications.
Based on the reviewed literature, future possible research
opportunities were proposed, providing a meaningful
guideline for researchers, government, and building owners
to develop feasible swimming pool heating systems. The
proposed research opportunities were shown as follows:
Although mathematical models for describing the heat
transfer process in swimming pools have been proposed,
the precondition of using this model is the assumption
that one temperature point is regarded as the water tem-
perature of the entire pool. In fact, the water temperature
varies in different regions of the pool. For example, in
the OSP model, shading of the building will affect the
solar energy that different regions of the pool can obtain.
Therefore, future research is necessary on the influence
of temperature distribution in swimming pools on heating
system control and thermal comfort.
Few studies have presented the heat gained from the
bodies of swimmers in the heat transfer model. The
activities and number of swimmers will affect the heat
exchange between swimmers and the pool water. For ISPs,
the activities and number of swimmers will also affect
the heating load of the facilities. Hence, it is suggested that
the effect of the heat gain from the bodies of swimmers
on the heat transfer model should be investigated in
The energy consumption of swimming pool facilities is
affected by many factors, such as location, climate, and
operating time. Current surveys for collecting data about
the energy consumption of swimming pool facilities are
limited. More surveys or simulation studies should be
conducted to investigate this. Necessary comparisons
between ISPs and OSPs with similar dimensions and
same operating conditions might be made.
In the current studies, a thermal-insulation cover was
placed on the surface of the pool when it was closed. This
could prevent heat loss from the surface, such as evaporative
and convective heat loss. However, thermal-insulation
covers should also be used on the walls and bottom of the
pool, contributing to reducing conductive heat loss during
the pool’s both open and closed periods.
During a pool’s unoccupied period, heating applications
such as solar collectors and heat pumps might not be
used to provide heat for the pool. Thermal energy storage
technologies such as PCM should be used to store the
heat from these applications. For solar collectors, the heat
can be stored in thermal energy storage devices during
the period when solar irradiation is adequate, and no heat
is needed for the pool. For heat pumps, the heat can be
stored in thermal energy devices during the period when
the electricity price is favorable, and no heat is needed for
Current investigations into the use of geothermal energy
technology in both ISP and OSP heating systems have
mainly focused on an initial evaluation of the system.
They have analyzed the performance of the system based
on simple calculations. Suitable design and control
strategies for systems with geothermal energy technology
are still lacking. In addition, it is still necessary to consider
the issue of maintaining the long-term operation of systems
with geothermal energy technology. Initial and maintenance
costs of a system with geothermal energy technology
were high for maintaining long-term operation. Hence,
more investigations should be conducted, to enhance the
economic benefits of this system.
Most swimming pool heating technologies consider the use
of solar energy and heat pumps. However, more attention
should be focused on other efficient techniques, such as
geothermal energy and biomass energy technologies.
Many scholars only propose a concept for heating
technologies in swimming pools and perform a simple
Li et al. / Building Simulation
feasibility analysis for the proposed system. Details about
the proper control and operation of the proposed systems
are lacking, because they may have a significant effect on
the real achieved results.
A few studies presented the optimization and optimal
control for the studied swimming pool heating systems.
However, multi-objective optimizations should be perfor-
med, considering multi-aspects, such as energy, economics,
exergy, thermal comfort, and environmental performance.
Due to abundant solar irradiance and high ambient tem-
perature in summer, the water temperature of the OSP
might be high, even beyond the thermal comfortable range
of swimmers. It is necessary to use cooling techniques
to cool the pool water down, and the simplest technique
might be to use the cooling capacity of the fresh cold
water. The extra heat of the OSP can be used in the aspects
of domestic hot water and showering.
The work described in this paper was supported by a grant
from the Research Grants Council of the Hong Kong Special
Administrative Region, China (No. 11208918). The authors
appreciated the support of funding from the Department
of Energy and Process Engineering of the Norwegian
University of Science and Technology, Norway.
Funding note: Open access funding provided by NTNU
Norwegian University of Science and Technology (incl St.
Olavs Hospital - Trondheim University Hospital).
Open Access: This article is licensed under a Creative
Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and repro-
duction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate
if changes were made.
The images or other third party material in this article
are included in the article’s Creative Commons licence,
unless indicated otherwise in a credit line to the material. If
material is not included in the article’s Creative Commons
licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to
obtain permission directly from the copyright holder.
To view a copy of this licence, visit
Abdel-Khalik SI (1976). Heat removal factor for a flat-plate solar
collector with a serpentine tube. Solar Energy, 18: 59–64.
Aketouane Z, Malha M, Bruneau D, Bah A, Michel B, Asbik M,
Ansari O (2018). Energy savings potential by integrating Phase
Change Material into hollow bricks: The case of Moroccan
buildings. Building Simulation, 11: 1109–1122.
Alkhamis A, Sherif S (1992). Performance analysis of a solar-assisted
swimming pool heating system. Energy, 17: 1165–1172.
Amin ZM, Hawlader MNA (2013). A review on solar assisted heat
pump systems in Singapore. Renewable and Sustainable Energy
Reviews, 26: 286–293.
Argyrou MC, Christodoulides P, Kalogirou SA (2018). Energy storage
for electricity generation and related processes: Technologies
appraisal and grid scale applications. Renewable and Sustainable
Energy Reviews, 94: 804–821.
ASHRAE (2003). ASHRAE Handbook: HVAC Applications. Atlanta:
American Society of Heating, Refrigerating and Air-Conditioning
Asdrubali F (2009). A scale model to evaluate water evaporation from
indoor swimming pools. Energy and Buildings, 41: 311–319.
Bai Y, Chow T-T, Ménézo C, Dupeyrat P (2012). Analysis of a hybrid
PV/thermal solar-assisted heat pump system for sports center
water heating application. International Journal of Photoenergy,
Bajwa DS, Peterson T, Sharma N, Shojaeiarani J, Bajwa SG (2018). A
review of densified solid biomass for energy production. Renewable
and Sustainable Energy Reviews, 96: 296–305.
Barbato M, Cirillo L, Menditto L, Moretti R, Nardini S (2018). Feasibility
study of a geothermal energy system for indoor swimming pool
in Campi Flegrei area. Thermal Science and Engineering Progress,
Bazilian MD, Leenders F, van der Ree B, Prasad D (2001). Photovoltaic
cogeneration in the built environment. Solar Energy, 71: 57–69.
Berardi U, Soudian S (2018). Benefits of latent thermal energy storage
in the retrofit of Canadian high-rise residential buildings. Building
Simulation, 11: 709–723.
Bergman TL, Incropera FP, DeWitt DP, Lavine AS (2011). Fundamentals
of Heat and Mass Transfer, 7th edn. Hoboken, NJ, USA: John
Wiley & Sons.
Blázquez JLF, Maestre IR, Gallero FJG, Gómez PÁ (2017). A new
practical CFD-based methodology to calculate the evaporation rate
in indoor swimming pools. Energy and Buildings, 149: 133–141.
Blázquez JLF, Maestre IR, González Gallero FJ, Álvarez Gómez P
(2018). Experimental test for the estimation of the evaporation
rate in indoor swimming pools: Validation of a new CFD-
based simulation methodology. Building and Environment, 138:
Borge D, Colmenar A, Castro M, Martín S, Sancristobal E (2011).
Exergy efficiency analysis in buildings climatized with LiCl–H2O
solar cooling systems that use swimming pools as heat sinks.
Energy and Buildings, 43: 3161–3172.
Bourne S, Novoselac A (2015). Compact PCM-based thermal stores
for shifting peak cooling loads. Building Simulation, 8: 673–688.
Bowen IS (1926). The ratio of heat losses by conduction and by
evaporation from any water surface. Physical Review, 27: 779.
Brambley MR, Wells SE (1983). Energy-conservation measures for
indoor swimming pools. Energy, 8: 403–418.
Li et al. / Building Simulation
Brook F (1955). Solar Energy Research. Daniels F, Duffle JA (eds).
Madison: University of Wisconsin Press.
Buonomano A, de Luca G, Figaj RD, Vanoli L (2015). Dynamic
simulation and thermo-economic analysis of a PhotoVoltaic/
Thermal collector heating system for an indoor–outdoor swimming
pool. Energy Conversion and Management, 99: 176–192.
Chan WW, Lam JC (2003). Energy-saving supporting tourism
sustainability: A case study of hotel swimming pool heat pump.
Journal of Sustainable Tourism, 11: 74–83.
Chow TT, Bai Y, Fong KF, Lin Z (2012). Analysis of a solar assisted
heat pump system for indoor swimming pool water and space
heating. Applied Energy, 100: 309–317.
Ciuman P, Lipska B (2018). Experimental validation of the numerical
model of air, heat and moisture flow in an indoor swimming
pool. Building and Environment, 145: 1–13.
Clot MR, Rosa-Clot P, Tina GM (2017). Submerged PV solar panel
for swimming pools: SP3. Energy Procedia, 134: 567–576.
Croy R, Peuser FA (1994). Experience with solar systems for heating
swimming pools in Germany. Solar Energy, 53: 47–52.
Cunio LN, Sproul AB (2012). Performance characterisation and energy
savings of uncovered swimming pool solar collectors under
reduced flow rate conditions. Solar Energy, 86: 1511–1517.
Czarnecki JT (1963). A method of heating swimming pools by solar
energy. Solar Energy, 7: 3–7.
Dang A (1986). A parametric study of swimming pool heating—I.
Energy Conversion and Management, 26: 27–31.
Degaulle (2020a). A project for a Vietnam hotel. Available at https://
Degaulle (2020b). A project for an Indonesia hotel. Available at
Delmastro C, Mutani G, Schranz L (2015). Advantages of coupling a
woody biomass cogeneration plant with a district heating network
for a sustainable built environment: A case study in luserna San
Giovanni (Torino, Italy). Energy Procedia, 78: 794–799.
Drissi Lamrhari E-H, Benhamou B (2018). Thermal behavior and
energy saving analysis of a flat with different energy efficiency
measures in six climates. Building Simulation, 11: 1123–1144.
Du Y, Mak CM, Liu J, Xia Q, Niu J, Kwok KCS (2017). Effects of
lift-up design on pedestrian level wind comfort in different
building configurations under three wind directions. Building
and Environment, 117: 84–99.
Du Y, Mak CM, Ai Z (2018). Modelling of pedestrian level wind
environment on a high-quality mesh: A case study for the HKPolyU
campus. Environmental Modelling & Software, 103: 105–119.
Du R, Li W, Xiong T, Yang X, Wang Y, Shah KW (2019). Numerical
investigation on the melting of nanoparticle-enhanced PCM in
latent heat energy storage unit with spiral coil heat exchanger.
Building Simulation, 12: 869–879.
Du Y, Blocken B, Pirker S (2020). A novel approach to simulate
pollutant dispersion in the built environment: Transport-based
recurrence CFD. Building and Environment, 170: 106604.
EU (2020). EU 2030 climate & energy framework. Available at
Facão, J, Oliveira AC (2006). Analysis of a micro-cogeneration system
using hybrid solar/gas collectors. International Journal of Low-
Carbon Technologies, 1: 285–297.
Feng X, Yan D, Yu R, Gao Y (2017). Investigation and modelling of
the centralized solar domestic hot water system in residential
buildings. Building Simulation, 10: 87–96.
Francey JLA, Golding P, Clarke R (1980). Low-cost solar heating of
community pools using pool covers. Solar Energy, 25: 407–416.
Furukakoi M, Adewuyi OB, Matayoshi H, Howlader AM, Senjyu T
(2018). Multi objective unit commitment with voltage stability
and PV uncertainty. Applied Energy, 228: 618–623.
Gorzin M, Hosseini MJ, Ranjbar AA, Bahrampoury R (2018).
Investigation of PCM charging for the energy saving of domestic
hot water system. Applied Thermal Engineering, 137: 659–668.
Govaer D, Zarmi Y (1981). Analytical evaluation of direct solar heating
of swimming pools. Solar Energy, 27: 529–533.
Govaer D (1984). Determining the solar heating of swimming pools
by the utilizability method. Solar Energy, 32: 667–669.
Greyvenstein GP, Meyer JP (1991). The viability of heat pumps for the
heating of swimming pools in South Africa. Energy, 16: 1031–1037.
Haaf W, Luboschik U, Tesche B (1994). Solar swimming pool heating:
Description of a validated model. Solar Energy, 53: 41–46.
Hahne E, Kübler R (1994). Monitoring and simulation of the thermal
performance of solar heated outdoor swimming pools. Solar
Energy, 53: 9–19.
Hanssen S, Mathisen H (1990). Evaporation from swimming pools. In:
Proceedings of the 2nd International Conference on Engineering
Aero-and Thermodynamics of Ventilated Rooms (Roomvent’90),
Harkouss F, Fardoun F, Biwole PH (2018). Optimization approaches
and climates investigations in NZEB—A review. Building Simulation,
Harrington C, Modera M (2013). Swimming pools as heat sinks for
air conditioners: California feasibility analysis. Energy and Buildings,
Holmen (2020). Holmen indoor Swimming Pool, Asker. Available at
Hong T, Langevin J, Sun K (2018). Building simulation: Ten challenges.
Building Simulation, 11: 871–898.
Hou J, Cao M, Liu P (2018). Development and utilization of geothermal
energy in China: Current practices and future strategies. Renewable
Energy, 125: 401–412.
ISO/TC 180 (1995). ISO/TC 180/SC 4 N 140. Solar Energy—Heating
Systems for Swimming Pools—Design and Installation.
Johansson L, Westerlund L (2001). Energy savings in indoor
swimming-pools: comparison between different heat-recovery
systems. Applied Energy, 70: 281–303.
Kampel W, Aas B, Bruland A (2013). Energy-use in Norwegian
swimming halls. Energy and Buildings, 59: 181–186.
Kampel W, Carlucci S, Aas B, Bruland A (2016). A proposal of energy
performance indicators for a reliable benchmark of swimming
facilities. Energy and Buildings, 129: 186–198.
Katsaprakakis DA (2015). Comparison of swimming pools alternative
passive and active heating systems based on renewable energy
sources in Southern Europe. Energy, 81: 738–753.
Kim S, Lee H, Kim H, Jang D-H, Kim H-J, Hur J, Cho Y-S, Hur K
(2018). Improvement in policy and proactive interconnection
procedure for renewable energy expansion in South Korea.
Renewable and Sustainable Energy Reviews, 98: 150–162.
Li et al. / Building Simulation
Kuyumcu ME, Tutumlu H, Yumrutaş R (2016). Performance of a
swimming pool heating system by utilizing waste energy rejected
from an ice rink with an energy storage tank. Energy Conversion
and Management, 121: 349–357.
Lam JC, Chan WW (2001). Life cycle energy cost analysis of heat
pump application for hotel swimming pools. Energy Conversion
and Management, 42: 1299–1306.
Lam JC, Chan WW (2003). Energy performance of air-to-water and
water-to-water heat pumps in hotel applications. Energy Conversion
and Management, 44: 1625–1631.
Lan S, Yang Z, Chen R, Stobart R (2018). A dynamic model for
thermoelectric generator applied to vehicle waste heat recovery.
Applied Energy, 210: 327–338.
Lazzarin RM, Longo GA (1996). Comparison of heat recovery systems
in public indoor swimming pools. Applied Thermal Engineering,
Lee W-S, Kung C-K (2008). Optimization of heat pump system in
indoor swimming pool using particle swarm algorithm. Applied
Thermal Engineering, 28: 1647–1653.
Li Y, Zhang Q, Sun X, Du Y, Liao S (2015). Optimization on performance
of the latent heat storage unit (LHSU) in telecommunications
base stations (TBSs) in China. Energy Procedia, 75: 2119–2124.
Li Y, Du Y, Xu T, Wu H, Zhou X, Ling Z, Zhang Z (2018a). Optimization
of thermal management system for Li-ion batteries using phase
change material. Applied Thermal Engineering, 131: 766–778.
Li Y, Huang G, Wu H, Xu T (2018b). Feasibility study of a PCM
storage tank integrated heating system for outdoor swimming
pools during the winter season. Applied Thermal Engineering,
Li Y, Huang G, Xu T, Liu X, Wu H (2018c). Optimal design of PCM
thermal storage tank and its application for winter available
open-air swimming pool. Applied Energy, 209: 224–235.
Li Y, Huang G (2019). Development of an integrated low-carbon
heating system for outdoor swimming pools for winter application.
In: Proceedings of the 13th REHVA World Congress (CLIMA
Li Z, Dai J, Chen H, Lin B (2019). An ANN-based fast building energy
consumption prediction method for complex architectural form
at the early design stage. Building Simulation, 12: 665–681.
Li Y, Ding Z, Du Y (2020a). Techno-economic optimization of
open-air swimming pool heating system with PCM storage tank
for winter applications. Renewable Energy, 150: 878–890.
Li Y, Ding Z, Shakerin M, Zhang N (2020b). A multi-objective optimal
design method for thermal energy storage systems with PCM: A
case study for outdoor swimming pool heating application. Journal
of Energy Storage, 29: 101371.
Li Y, Zhang N, Ding Z (2020c). Investigation on the energy performance
of using air-source heat pump to charge PCM storage tank.
Journal of Energy Storage, 28: 101270.
Li C, Li C, Lyu Y, Qiu Z (2020d). Performance of double-circulation
water-flow window system as solar collector and indoor heating
terminal. Building Simulation, 13: 575–584.
Limane A, Fellouah H, Galanis N (2017). Simulation of airflow with
heat and mass transfer in an indoor swimming pool by OpenFOAM.
International Journal of Heat and Mass Transfer, 109: 862–878.
Limane A, Fellouah H, Galanis N (2018). Three-dimensional OpenFOAM
simulation to evaluate the thermal comfort of occupants, indoor
air quality and heat losses inside an indoor swimming pool.
Energy and Buildings, 167: 49–68.
Lu T, Lü X, Viljanen M (2014). Prediction of water evaporation rate
for indoor swimming hall using neural networks. Energy and
Buildings, 81: 268–280.
McKeown D (2009). Swimming Pool Operator’s Manual. Toronto:
McMillan W (1971). Heat dispersal—Lake trawsfynydd cooling studies.
In: Symposium on Freshwater Biology and Electrical Power
Generation, Part I, Session 1, pp. 41–80.
MEPC (2020). Typical material used in thermal-insulation covers.
Available at https://www.m-ep.co.jp/.
Mohanraj M, Belyayev Y, Jayaraj S, Kaltayev A (2018). Research and
developments on solar assisted compression heat pump systems—A
comprehensive review (Part A: Modeling and modifications).
Renewable and Sustainable Energy Reviews, 83: 90–123.
Molineaux B, Lachal B, Guisan O (1994a). Thermal analysis of five
outdoor swimming pools heated by unglazed solar collectors.
Solar Energy, 53: 21–26.
Molineaux B, Lachal B, Guisan O (1994b). Thermal analysis of five
unglazed solar collector systems for the heating of outdoor
swimming pools. Solar Energy, 53: 27–32.
Mousia A, Dimoudi A (2015). Energy performance of open air
swimming pools in Greece. Energy and Buildings, 90: 166–172.
Moya D, Aldás C, Kaparaju P (2018). Geothermal energy: Power plant
technology and direct heat applications. Renewable and Sustainable
Energy Reviews, 94: 889–901.
OMNEXUS (2020). Typical material used in thermal-insulation
covers. Available at https://omnexus.specialchem.com/.
Oró E, Allepuz R, Martorell I, Salom J (2018). Design and economic
analysis of liquid cooled data centres for waste heat recovery: A
case study for an indoor swimming pool. Sustainable Cities and
Society, 36: 185–203.
Panaras G, Markogiannaki M, Tolis EI, Sakellaris Y, Bartzis JG (2018).
Experimental and theoretical investigation of air exchange rate of
an indoor aquatic center. Sustainable Cities and Society, 39: 126–134.
Panchabikesan K, Joybari MM, Haghighat F, Ramalingam V, Ding Y
(2020). Feasibility study on the year-round operation of PCM
based free cooling systems in tropical climatic conditions. Energy,
Rajagopalan P, Jamei E (2015). Thermal comfort of multiple user groups
in indoor aquatic centres. Energy and Buildings, 105: 129–138.
Rakopoulos CD, Vazeos E (1987). A model of the energy fluxes in a
solar heated swimming pool and its experimental validation.
Energy Conversion and Management, 27: 189–195.
Richter D (1979). Temperatur-und Wärmehaushalt des thermisch
belasteten Stechlin-und Nehmitzsees Abhandlung des
Meteorologischen Dienstes der DDR Nr. 123. Berlin: Akademie-
Rohwer C (1931). Evaporation from free water surfaces. Technical
Bulletins 163103. US Department of Agriculture.
Romberg TA (1995). Reform in School Mathematics and Authentic
Assessment. Albany, NY, USA: State University of New York Press.
Li et al. / Building Simulation
Root DE, Jr (1960). Practical aspects of solar swimming pool heating.
Solar Energy, 4: 23–24.
Ruiz E, Martínez PJ (2010). Analysis of an open-air swimming
pool solar heating system by using an experimentally validated
TRNSYS model. Solar Energy, 84: 116–123.
Sakah M, Diawuo FA, Katzenbach R, Gyamfi S (2017). Towards a
sustainable electrification in Ghana: A review of renewable energy
deployment policies. Renewable and Sustainable Energy Reviews,
Shah MM (2002). Evaluation of available correlations for rate of
evaporation from undisturbed water pools to quiet air. HVAC&R
Research, 8: 125–131.
Shah MM (2003). Prediction of evaporation from occupied indoor
swimming pools. Energy and Buildings, 35: 707–713.
Shah MM (2012). Improved method for calculating evaporation from
indoor water pools. Energy and Buildings, 49: 306–309.
Shah M (2014). Methods for calculation of evaporation from swimming
pools and other water surfaces. ASHRAE Transactions, 120(2):
Sharifi A, Yamagata Y (2015). Roof ponds as passive heating and
cooling systems: A systematic review. Applied Energy, 160: 336–357.
Singh M, Tiwari GN, Yadav YP (1989). Solar energy utilization
for heating of indoor swimming pool. Energy Conversion and
Management, 29: 239–244.
Smith CC, Löf G, Jones R (1994). Measurement and analysis of
evaporation from an inactive outdoor swimming pool. Solar
Energy, 53: 3–7.
Somwanshi A, Tiwari AK, Sodha MS (2013). Feasibility of earth heat
storage for all weather conditioning of open swimming pool water.
Energy Conversion and Management, 68: 89–95.
Sun P, Wu J, Wang R, Xu Y (2011). Analysis of indoor environmental
conditions and heat pump energy supply systems in indoor
swimming pools. Energy and Buildings, 43: 1071–1080.
Szeicz G, McMonagle RC (1983). The heat balance of urban swimming
pools. Solar Energy, 30: 247–259.
Tagliafico LA, Scarpa F, Tagliafico G, Valsuani F (2012). An approach
to energy saving assessment of solar assisted heat pumps for
swimming pool water heating. Energy and Buildings, 55: 833–840.
Tiwari GN, Sharma SB (1991). Design parameters for indoor
swimming-pool heating using solar energy. Energy, 16: 971–975.
Tong LG, Zhang P, Yin SW, Zhang PK, Liu CP, et al. (2018). Waste
heat recovery method for the air pre-purification system of an
air separation unit. Applied Thermal Engineering, 143: 123–129.
Trianti-Stourna E, Spyropoulou K, Theofylaktos C, Droutsa K, Balaras
CA, et al. (1998). Energy conservation strategies for sports centers:
Part B. Swimming pools. Energy and Buildings, 27: 123–135.
Uribe D, Bustamante W, Vera S (2018). Potential of perforated exterior
louvers to improve the comfort and energy performance of an
office space in different climates. Building Simulation, 11: 695–708.
Varming (2020). Difference between passive and active applications.
Available at http://www.varming.ie/blog/the-difference-between-
Wang K, Herrando M, Pantaleo AM, Markides CN (2019).
Thermoeconomic assessment of a PV/T combined heating and
power system for University Sport Centre of Bari. Energy Procedia,
Westerlund L, Dahl J (1994). Use of an open absorption heat-pump
for energy conservation in a public swimming-pool. Applied
Energy, 49: 275–300.
Winterton RHS (1999). Newton’s law of cooling. Contemporary Physics,
Woolley J, Harrington C, Modera M (2011). Swimming pools as heat
sinks for air conditioners: Model design and experimental
validation for natural thermal behavior of the pool. Building and
Environment, 46: 187–195.
Wu H, Tang R, Li Z, Zhong H (2009). A mathematical procedure
to estimate solar absorptance of shallow water ponds. Energy
Conversion and Management, 50: 1828–1833.
Xie H, Gong G, Fu M, Wang P, Li L (2018). A thermodynamic
method to calculate energy & exergy consumption and CO2
emission of building materials based on economic indicator.
Building Simulation, 11: 235–244.
Xu T, Li Y, Chen J, Liu J (2017). Preparation and thermal energy
storage properties of LiNO3-KCl-NaNO3/expanded graphite
composite phase change material. Solar Energy Materials and
Solar Cells, 169: 215–221.
Xu T, Li Y, Chen J, Wu H, Zhou X, Zhang Z (2018). Improving
thermal management of electronic apparatus with paraffin (PA)/
expanded graphite (EG)/graphene (GN) composite material.
Applied Thermal Engineering, 140: 13–22.
Yadav YP, Tiwari GN (1987). Analytical model of solar swimming pool:
Transient approach. Energy Conversion and Management, 27:
Yandri E (2017). The effect of Joule heating to thermal performance
of hybrid PVT collector during electricity generation. Renewable
Energy, 111: 344–352.
Zhao J, Bilbao JI, Spooner ED, Sproul AB (2018). Experimental study
of a solar pool heating system under lower flow and low pump
speed conditions. Renewable Energy, 119: 320–335.
Zsembinszki G, Farid MM, Cabeza LF (2012). Analysis of implementing
phase change materials in open-air swimming pools. Solar Energy,