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Energy Sources, Part B: Economics, Planning, and Policy
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/uesb20
Solar seasonal thermal energy storage for space
heating in residential buildings: Optimization and
comparison with an air-source heat pump
Jie Lu , Guoqing He & Feng Mao
To cite this article: Jie Lu , Guoqing He & Feng Mao (2020): Solar seasonal thermal energy
storage for space heating in residential buildings: Optimization and comparison with an
air-source heat pump, Energy Sources, Part B: Economics, Planning, and Policy, DOI:
10.1080/15567249.2020.1786192
To link to this article: https://doi.org/10.1080/15567249.2020.1786192
Published online: 20 Jul 2020.
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Solar seasonal thermal energy storage for space heating in
residential buildings: Optimization and comparison with an
air-source heat pump
Jie Lu
a
, Guoqing He
a
, and Feng Mao
b
a
College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, China;
b
Department of Marketing,
Hangzhou Huadian Huayuan Environment Engineering Co.,Ltd, Hangzhou, China
ABSTRACT
This study evaluates the techno-economics of replacing an air-source heat
pump (ASHP) system with a solar seasonal thermal energy storage (STES)
system for space heating in Hangzhou, China. Three heating systems, solar
STES, ASHP, and ASHP with short-term storage of solar energy, are devel-
oped using TRNSYS for a house with 240 m
2
of oor area. The ratio of tank
volume to collector area (RVA) of the STES is optimized for the lowest
equivalent annual cost over a lifespan of 20 y. The determined optimal
RVA is 0.33 m
3
/m
2
, although it depends on the system and electricity prices.
The optimized STES reduces the electricity demand to 1,269 kWh (74%
reduction). Despite the superior energy performance, the economic benet
is only possible with large STES systems, which enjoy low tank prices due to
scale eects. The results suggest that policy support is needed for STES,
where district scaling is not an option.
KEYWORDS
Air-source heat pump;
economic performance;
optimization; ratio of volume
to area; seasonal thermal
energy storage; tank
1. Introduction
Solar thermal utilization is a technically mature renewable solution for the supply of domestic hot water
in the global effort to reduce fossil fuel consumption (Hepbasli, Ulgen, and Eke 2004; Hovsepian and
Kaiser 1997). Despite the significant growth in installations over the past couple of decades, the
penetration of solar thermal systems still faces substantial economic and social barriers (Hang, Qu,
and Zhao 2012; Leckner and Zmeureanu 2011; Martinopoulos and Tsalikis 2018; Sovacool and
Martiskainen 2020; Urmeea et al. 2018). Among these barriers is the lower cost of electricity (Azeez
and Atikol 2019). Recently, the use of solar thermal energy in space heating has been attracting attention
(Huang, Fan, and Furbo 2019; Marcos, Izquierdo, and Parra 2011; Martinopoulos and Tsalikis 2014),
especially in northern Europe, where district heating is increasing and seasonal energy storage technol-
ogy has been developed to improve the energy performance of solar thermal systems (Dahash et al. 2019).
In the hot summer cold winter (HSCW) region of China, domestic space heating is a fast-growing
market (Zhang, Zheng, and Huang 2014), and solar energy is considered an appealing alternative to
cope with air pollution and energy pressure (Huang and Huang 2017). Local governments have
planned to increase the share of renewable energy in the heating market (Huang, Fan, and Furbo
2019). Incentive policies and regulations have been made to promote building designs that incorporate
solar energy and heat pumps (He et al. 2015).
Solar energy would be an ideal heating energy source except for two issues: 1) It is least abundant
in winter when it is needed most. 2) Its energy density is low, and substantial investment in solar
CONTACT Guoqing He Guoqinghe@zju.edu.cn College of Civil Engineering and Architecture, Zhejiang University,
Hangzhou, China
This article has been republished with minor changes. These changes do not impact the academic content of the article.
ENERGY SOURCES, PART B: ECONOMICS, PLANNING, AND POLICY
https://doi.org/10.1080/15567249.2020.1786192
© 2020 Taylor & Francis Group, LLC
collectors is required. Seasonal thermal energy storage (STES) systems appear to be a promising
solution to these issues by storing excessive summer solar energy in rocks, soil, aquifers, or water
tanks for use in winter (Dahash et al. 2019; Hesaraki, Halilovic, and Holmberg 2015). To date, many
sizable central heating systems have been constructed, mostly in Europe (Bokhoven and Van Dam
2001; Jradi, Veje, and Jørgensen 2017; Ochs, Heidemann, and Müller-Steinhagen 2009; Paksoy et al.
2000) and in other countries as well (Chung, Park, and Yoon 1998; Krupczak et al. 1986; Li et al.
2015; Walton and McSwiggen 1983; Zhang et al. 2015). Most STES projects store energy in the form
of sensible heat (mostly water), while latent heat and chemical methods are considered promising
but are not yet mature (Fatih Demirbas 2006; Xu and Wang 2019). In cold climates, the borehole
system assisted with a heat pump is recommended (Shah, Aye, and Rismanchi 2018). However, due
to the high water table level in HSCW regions, STES systems with a water tank or pit are probably
a more suitable choice. Therefore, this study focuses on tank STES systems.
For tank systems, the initial investment is dependent mainly on the size of the tank and solar
collectors. Their proper sizing is critical not only for energy capacity but also for economic perfor-
mance (McKenna, Fehrenbach, and Merkel 2019). In the literature, the ratio of the storage volume to
the collector area (RVA, m
3
/m
2
) has received much attention. Much discussion of this value has been
energy-oriented with a special focus on the energy efficiency or energy capacity of the storage system,
leading to relatively large storage systems. Guadalfajara et al. (2015) proposed an RVA value of 6.1 m
3
/
m
2
to ensure no rejection of collected solar energy. In other studies (Durão, Joyce, and Mendes 2014;
Hesaraki, Halilovic, and Holmberg 2015), a similar ratio (5 m
3
/m
2
) was recommended for maximum
efficiency. From the review work by Dahash et al. (2019), most large-scale district heating projects
constructed in Germany, Denmark, and several other European countries in the last two decades have
a ratio greater than 1 m
3
/m
2
and some as high as 5.6 m
3
/m
2
. Smaller RVA values, however, also have
been suggested for domestic heating systems. Li et al. (2014) proposed that a storage factor of
0.567 m
3
/m
2
was most suitable for a system in Beijing considering the balance between storage
efficiency and solar collector efficiency. Ma et al. (2018) calculated the preferred RVA as ranging
from 0.67 to 1.0 m
3
/m
2
for residential houses in the UK, assuming that the STES system met 100% of
the heating demand. Beausoleil-Morrison et al. (2019) recommended a 36 m
3
tank and a 41.6 m
2
collector (RVA = 0.87 m
3
/m
2
) for a residential project.
However, all RVA values from projects adopting seasonal storage are still an order of magnitude
higher than those without considering seasonal storage (Hang, Qu, and Zhao 2012; Li et al. 2015;
Marcos, Izquierdo, and Parra 2011). For example, for a tank of 0.3 m
3
, Leckner and Zmeureanu
(2011) found that a collector area of 11 m
2
was best to maximize the energy reduction for a house in
Canada. In Greece, Martinopoulos and Tsalikis (2014) suggested that from an economic point of
view, 12 m
2
of collectors with a tank of 0.5 or 0.65 m
3
is the best combination for energy-efficient
houses in four cities.
Thus far, to the best of our knowledge, few studies have discussed the optimization of the
STES RVA based on economic performance. Launay et al. (2019) presented an optimal analysis of
a residential project using multiple criteria, including solar fraction and the Levelized Cost of
energy; however, they did not provide an optimized sizing for the minimum Levelized Cost of the
energy. Although STES projects may still need policy support (Milewski, Wołowicz, and Bujalski
2013; Renaldi and Friedrich 2019), the economic aspect of STES is an essential factor in the
decision-making process. The economic performance will depend on climate, geological condi-
tions, local market conditions, and policies. Unlike most studies in the literature where the cost-
effectiveness of the STES is evaluated against conventional energy sources, such as natural gas or
electricity, this study analyzes the cost-effectiveness of the STES system through comparison with
the popular ASHP heating system, which is accepted as an energy-efficient system (Li et al. 2007;
Liu et al. 2016) in the HSCW region. The objective is to evaluate the economic feasibility of
replacing the ASHP with a solar STES to reduce the growing rate of electricity demand in the
heating market. The results will help policymakers and designers to incorporate the best of this
technology in this region.
2J. LU ET AL.
2. Methodology
The scope of the problem is to provide space heating for residential buildings in Hangzhou city in the
HSCW region using as much solar energy as possible with economic performance as a constraint.
2.1. Building and site
Hangzhou city (30°16ʹN, 120°12ʹE) lies in the lower basin of the Yangtze River. Its climate produces
a strong demand for both cooling (summer) and heating (winter). Based on the Typical
Meteorological Year data provided by the China Meteorological Administration and Tsinghua
University (2005), the daily average temperature varies between – 5°C and 37°C and the average
total horizontal solar irradiation is 13.5 MJ/(m
2
∙day). The variations of the solar radiation and ambient
temperature are plotted in Figure 6(a,b), respectively. In this study, a heating period of 120 days
(2880 h) from November 15 to March 15 of the next year was considered.
The heating demand of an energy-efficient two-story house with 240 m
2
floor area was used.
It is a 12 m wide and 10 m long rectangular-shaped building with a tilted rooftop for solar
collector installation. Note that the building model represents a low-energy building with a well-
insulated envelope. The dynamic heating demand was simulated using the TRNSYS software
(Klein and Beckman 2004) at a time step of 15 min. The weather data file was imported as an
external file in Type 109 data reader in the standard TMY2 format. The building thermal load
was modeled either using the Type 12 c model if the floor heating system was used or the Type
88 model if fan coil units were used. The Type 14 c component was selected to account for the
internal gains from the occupants, appliances, and lighting equipment. The calculated total
heating demand is 11,361 kWh (or 48 kWh/m
2
), and the maximum heating power is 25 kW.
In terms of heating demand per floor area, the house just meets the building energy efficiency
standard of 65% target of the leading cities in Northern China and the German WSVO’1995
standard (Zhou et al. 2018).
2.2. Systems and simulation
Three heating systems were designed and compared: the ASHP heating system, ASHP with solar water
heating (SWH) as the auxiliary heating source (ASHP + SWH), and STES + SWH system using
electrical heating as the auxiliary heating source. The water-source heat pump (WSHP) was used to
assist the STES+SWH system.
2.2.1. System A: STES+SWH system
System A is a STES+SWH system with WSHP working in series (Figure 1). The solar collector charges
the STES system all year long, which provides heat to the house during the heating period either
directly or via the WSHP.
A model representation in TRNSYS is illustrated in Figure 2. The system consists of three
subsystems, namely the solar collecting system, the storage system, and the floor heating system.
The solar collecting system consists of solar collectors (Type 1b), a circulating pump (Type 3), and
a controller (Type 2b) that manages the circulation of water between the collector and the storage
tank. The controller initiates circulation once the temperature at the outlet of the collectors
exceeds the temperature at the bottom of the tank by a set difference value and switches off the
pump if the exceedance is below another set difference value. Moreover, the controller continually
monitors the top node temperature of the tank for safety purposes and stops charging the tank
once its temperature reaches 95°C.
The seasonal storage tank is a buried, orthogonal-shaped-stratified tank (Type 534). To
model the stratified tank in TRNSYS, a previous study used five nodes to address the vertical
temperature change (Antoniadis and Martinopoulos 2019). In our study, eight nodes were
ENERGY SOURCES, PART B: ECONOMICS, PLANNING, AND POLICY 3
used as a result of balancing between accuracy and simulation speed. The size was deter-
mined such that the STES + WSHP system can meet 100% of the heating demand. The heat
exchange between layers is processed through mass flow and free convection. Thermal
Figure 1. System A: seasonal thermal energy storage (STES) + solar water heating (SWH) with a floor heating system.
Figure 2. Modeling of the seasonal thermal energy storage (STES)+solar water heating (SWH) system in TRNSYS.
4J. LU ET AL.
charging from the solar loop is realized through direct water exchange. The tank has an
average heat loss rate of 0.333 W/(m
2
∙K) to the surrounding soil. Soil temperature is a critical
parameter in the estimation of the thermal loss of the tank. This parameter is, however, often
not available, for which some simple methods have been used in the literature. For example,
Sweet and McLeskey (2012) used a yearly constant temperature of the soil, while Hesaraki
et al. (2015) used the mean annual outdoor temperature as the ground temperature. In this
study, the heat transfer between the tank and environment was simulated using the Type 707
object in TRNSYS with the dynamic temperature of the soil according to the weather data of
Hangzhou (Zhang, Wang, and Fu 2006).
The floor heating system was used such that a relatively low water supply temperature could
be used to match the SWH or heat pump heating systems. The Type 12 c was used to simulate
the building with floor heating systems for its excellent representation of the building heating
load and less calculation time (Antoniadis and Martinopoulos 2019). During the heating period,
the STES provides hot water directly to the building for space heating if the water temperature is
above 40°C. Otherwise, the water temperature is raised through the WSHP. Electrical heating
would serve the same purpose in terms of energy efficiency; however, the heat pump has a much
larger heating capacity for the same-rated power. A variable-speed compressor ensures that the
supply water temperature can be adjusted to match the heating demand. The heat pump has
a nominal coefficient of performance (COP) of 4. The WSHP stops when the temperature of the
storage tank falls below 5°C to prevent the formation of ice. The parameters used in TRNSYS are
shown in Table 1.
Table 1. System parameters used in TRNSYS modeling.
Model Parameter Value
Solar collectors
(Type 1)
Intercept efficiency, FRταð Þn, 0.8
Efficiency slope, FRUL, W/(m
2
∙K), 3.61
Installation slope, degrees 40
Soil property parameters
(Type 707)
a
Conductivity, W/(m∙K) 1.5
Density, kg/m
3
1500
Specific heat, kJ/(kg∙K) 2.2
Convection coefficient, W/(m
2
∙K) 17.78
Pump
(Type 3b)
Pump-1 (Solar pump) flow rate, kg/h 300
Pump-2 (Heating pump) flow rate, kg/h 100
WSHP pump flow rate, kg/h 500
Heating loop pump flow rate, kg/h 500
WSHP
(Type 225)
Rated heat capacity, kW 30
Rated COP 4
Outlet water temp, °C 45
Building
(Type 88 with fan coil heating units
or Type12 c with floor heating system)
Building capacitance, kJ/K 10000
Building volume, m
3
/s 720
Initial temperature, °C 15
Overall conductance of house, W/K 277.8
Latent heat ratio 0.23
Designed indoor temp, °C 20
Initial humidity, g/kg (Type 88) 5
Heat transfer rate of floor heating loop and the room,
kJ/(h∙K) (Type 12 c)
1200
Building loss coefficient, kJ/(h∙m
2
∙K) (Type 12 c) 4.2
ASHP
(Type 665–2)
Total air flow rate, 1/s 300.0
Rated indoor fan power, kJ/h 671.1
Rated outdoor fan power, kJ/h 745.7
Storage tank (Type 534) Average tank loss coefficient, W/(m
2
∙K) 0.33
Initial storage temperature in all layers, °C 20
Height-to-diameter ratio (HDR) 1
Number of tank nodes 8
Pumps
(Type 3b)
Pump-1 (Solar pump) flow rate, kg/h 300
Pump-2 (Heating pump) flow rate, kg/h 100
a
Data were taken from the reference (Zhang et al. 2006).
ENERGY SOURCES, PART B: ECONOMICS, PLANNING, AND POLICY 5
2.2.2. System B: ASHP heating system
The ASHP is very popular in this region. It is a cost-effective electricity-driven heating system that can
also provide cooling in summer. Figure 3 shows a schematic of this system. In practice, System B could
be a split air-conditioning unit or a variable refrigerant flow system. The parameters of the TRNSYS
model of the system are shown in Table 1.
2.2.3. System C: ASHP + SWH
System C is an ASHP with the addition of an SWH as an additional heating source that works in
parallel with the ASHP system. This SWH system has 40 m
2
flat plate solar collectors and a 2 m
3
tank
for short-term storage of solar energy (Figure 4). The parameters of the TRNSYS model of the system
are shown in Table 1.
The installation of the SWH for domestic hot water in urban buildings has been supported
by government subsidies and has now become mandatory in most provinces in this region
(He et al. 2015). However, the installation for space heating is still limited due to the high
initial cost.
Figure 3. System B: air-source heat pump (ASHP) with fan coil heating system.
Figure 4. System C: air-source heat pump (ASHP) with fan coil + solar water heater (SWH) for a floor heating system.
6J. LU ET AL.
2.3. Economic analysis method
Both the net present value (NPV) (Hovsepian and Kaiser 1997; Martinopoulos and Tsalikis 2014) and
the equivalent annual cost (EAC) (El-Bialy et al. 2016; Hirvonen, Ur Rehman, and Sirén 2018) have
been used in the literature for economic analysis. The two values serve similar purposes. Both are
based on a discount rate to evaluate an investment on an asset. The EAC is the annual cost of owning,
operating, and maintaining an asset over its entire life. The NPV can be treated as the present value of
owning and maintaining the heating system that generates an annuity equal to the amount of EAC
during the lifespan. In this study, the EAC was selected (Equation (1)) because its meaning is more
straightforward.
EAC ¼Ci1þið Þn
1þið Þn1þPE�EþαC (1)
where C is the initial investment in Chinese Yuan (CNY), EAC is the equivalent annual cost of the
system (CNY/y), i is the discount rate (interest rate or standard rate of return coefficient), n is the
lifespan of the system (y), P
E
is the electricity price (CNY/kWh), E is the total electrical demand
(kWh), α is the ratio of the annual maintenance cost to the initial cost. The first term on the right is the
average annual payment made to the bank for an initial loan amount of C. The remaining terms are
associated with operating costs. Typically, plate type collectors have a lower rate of maintenance cost
than vacuum collectors. In this study, α is 1%. The initial investment includes the equipment cost and
installation of labor for all system components. In the STES system, the cost can be estimated using
Equation (2).
C¼PC�AþPT�VþCWSHP þCP&P(2)
where the subscript P&P stands for pumps and pipework. P
T
is the cost of the storage tank (CNY/m
3
),
and P
C
is the price of collectors (CNY/m
2
). The determination of these values is shown in Table 2.
The high initial investment is attributable to the solar collector and the storage tank. The
current average price of collectors is P
C
= 1000 CNY/m
2
for flat plate types in China, including
pump and pipework (Li et al. 2018; Orosz et al. 2016; Sonsaree et al. 2018). Experiences show that
the unit cost can be reduced considerably for larger systems (Li and Zhu 2018;
Orosz and Mathaha 2016; Sonsaree et al. 2018). In an earlier study, Pfeil and Koch (2000)
compared the costs of two gravel/water thermal storage systems. They found that the cost of
the more extensive system (16,000 m
3
water equivalent) was only one-third of the smaller system
(1100 m
3
water equivalent). In this study, a unit cost of P
T
= 1867 CNY/m
3
was estimated for
a 40 m
3
underground cylinder-shaped concrete tank (Figure 5) with itemized costs listed in Table
3 based on current market values of labor and materials in China. For this type of storage tank,
the reinforced concrete, stainless steel, insulation, and waterproof materials are the most expen-
sive items. If the tank is a surface type, i.e., reinforced concrete is not used at the top regardless of
burying, it may cost approximately 1,350 CNY/m
3
. A plastic cylinder is much less expensive;
Table 2. Parameters used in the economic analysis (*1 CNY = 0.145188 USD).
Symbol Unit Calculation
WSHP C
WSHP
CNY 10,000
ASHP C
ASHP
CNY 65,000
Pump and pipework C
P&P
CNY 3,000
Electricity price P
E
CNY/kWh 0.518
Collector price P
C
CNY/m
2
1,000
Tank price P
T
CNY/m
3
1,867
Maintenance cost ratio α- 1%
Interest rate i- 8%
* Exchange ratio as of January 23, 2020, from China State Administration of Foreign
Exchange is http://www.safe.gov.cn/safe/index.html
ENERGY SOURCES, PART B: ECONOMICS, PLANNING, AND POLICY 7
however, the market availability of low-cost plastic containers that can store up to 80°C to 90°C
hot water cannot be ascertained. For large-scale storage systems, the unit cost of the storage
system can be reduced significantly. For example, it was estimated to be 924 CNY/m
3
for
a 5,274 m
3
tank constructed in the Jinke Heating and Power Plant in Inner Mongolia. In
Heilongjiang province, a similar tank of 9,500 m
3
in the Huadian Fulaerji Electric Heating and
Power Plant was constructed at the cost of approximately 800 CNY/m
3
. Therefore, it can be
assumed that the unit cost can be lower for a larger system.
3. Results and discussions
Energy performance is first discussed for the STES system with RVA = 40 m
3
/40 m
2
, followed by the
optimization of the STES system with economic constraints.
3.1. Energy performance (RVA = 40 m
3
/40 m
2
)
Figure 6(a) plots the temporal variations of the total daily solar radiation and daily average collector
efficiency. The collector efficiency is higher (40% to 60%) at the beginning of the charging period as
Figure 5. Schematic of an underground concrete water tank.
Table 3. Cost of an underground concrete tank (1 CNY = 0.145188 USD).
Composition Cost (CNY) Remark
Earth excavation 4,777 0.5 m soil on top
Backfilling soil 2,178
Slab cushion 1,513 Bottom framework of reinforced concrete slab for leveling
Reinforced concrete wall 21,419 D = 3.7 m, L = 3.7 m
Insulation 9,777 75 mm Polyurethane
Waterproof material 10,429 Sprayed polyurea to prevent water intrusion
Interior lining 24,811 2 mm stainless steel plate
Total cost 74,903
8J. LU ET AL.
well as in winter but is generally lower (<35%) during the summer and fall because of the increased
water temperature in the storage tank. The magnitude of the collector efficiency matches that observed
in actual projects (Jiao et al. 2015; Tao et al. 2015). Figure 6(b) shows that the dynamic indoor
temperature was comfortable during the heating period, with an average of 20.4°C. At the beginning of
the heating period, the indoor temperature fluctuates between the design value of 20°C and 23°C.
Over-heating is a result of a constant speed pump. Figure 6(c) shows the variations of the average
water temperature in the tank. The temperature increases at a relatively fast rate in the first month
until it reaches about 60°C, after which it increases at a much slower rate. The reason is that, at high
water temperatures, the collector efficiency drops quickly, and the heat loss of the tank increases. The
water temperature continues to increase until it reaches the highest value of 86.2°C on September 26.
Subsequently, it decreases due to relatively weaker solar radiation and lower ambient temperature. At
the beginning of the heating period (November 15), the temperature of the tank is 70.8°C.
Direct heating can last for approximately 43 days until December 28 when the water temperature
drops to 40°C. Thereafter, the WSHP is used to assist heating throughout the rest of the heating period
in a serial mode of a solar-heat pump system where the tank temperature generally remains low.
Figure 7 shows the energy flow in the whole system. Approximately one-third, or 16,488 kWh, of
the total incident solar energy is collected, almost half of which (7,035 kWh) is collected in winter.
Approximately 45% of the collected energy is lost to the environment through pipe works, pumps, and
soil contact. The solar fraction (ratio of the heating demand subtracted by the electricity demand to the
Figure 6. (a) Variation of the total daily solar radiation and daily average collector efficiency of the seasonal thermal energy storage
(STES) system; (b) Simulated variation of annual indoor temperature along with soil and air temperatures; (c) Simulated water
temperature in the tank.
ENERGY SOURCES, PART B: ECONOMICS, PLANNING, AND POLICY 9
heating demand) is 81%. The total electricity demand is 2,876 kWh and the system COP, the ratio of
total heating demand to the total electricity demand, is 3.95. At a similar RVA, the storage efficiency
(55%) and system COP are consistent with those of Li et al. (2014), and the solar fraction is consistent
with that of Beausoleil-Morrison et al. (2019). According to Beausoleil-Morrison et al. (2019),
increasing either the collector area or storage volume increases the solar fraction.
The energy performance of the three systems is compared in Figure 8. The COPs of systems B and
C are 2.9 and 3.5, respectively; both are lower than that of system A. Compared with system B, system
A reduces the electrical demand by approximately 40%. System C, which collects solar energy only in
winter, reduces the electrical demand by approximately 14%. The STES increases the energy capacity
of the collector (heating energy provided by collectors) from 51 kWh/m
2
in system C to 229 kWh/m
2
in system A, raising the solar fraction from 18% to 81%.
The calculated EAC costs are 9,861 CNY, 14,381 CNY, and 15,493 CNY for systems B, C, and A,
respectively, assuming a lifetime of 20 y for all systems. As expected, the ASHP is the most economical
Figure 7. Energy flow in the seasonal thermal energy storage (STES)+solar water heating (SWH) system and electricity amounts
(units: kWh).
Figure 8. Comparisons of the energy performance of three systems (STES = seasonal thermal energy storage; SWH = solar water
heater; ASHP = air-source heat pump).
10 J. LU ET AL.
solution, and adding a solar component solely for a winter application increases the cost by approxi-
mately 46%. Further addition of the seasonal storage system increases the annual cost further by 7.7%.
The loan interest appears to be the largest share in the annual cost. Comparing system A with system
B, the savings in energy cost, approximately 1,457 CNY/y, is far less than the increased annual cost in
paying off the loan interest (4,780 CNY/y). Note that a discount interest rate is not considered here,
although, in the literature, a discounted rate of 3% was adopted in the economic analyses (Hirvonen,
Ur Rehman, and Sirén 2018; Renaldi and Friedrich 2019). If the loan interest i and the maintenance
cost α are both zero, the EAC of system B (5,841 CNY/y) still remains significantly lower than that of
system A (7,369 CNY/y) or system B (7,658 CNY/y). This analysis indicates that the current high
prices of storage and collectors are the main factors preventing the STES system from competing
against the ASHP system.
3.2. Optimized RVA values for the lowest annual cost
The optimal RVA value for the lowest annual cost was found with the constraint that the STES +
WSHP system provides 100% of the heating demand. For any given tank volume, the minimum
collector area that meets this constraint can be determined. Figure 9 shows the line of the minimum
area along with the corresponding energy consumption and annual cost. As the line of the area moves
to the left, the system has a smaller storage tank but a large collector area, approaching that of system
C. As the line moves to the right, the system has a larger storage tank but a smaller collector area,
applying more weight on the solar energy from the non-heating season. The most economical choice
of sizes occurs at A = 66 m
2
and V = 20 m
3
, or RVA = 0.3 m
3
/m
2
. The corresponding annual cost is
13,360 CNY/y, a reduction of 14% compared with the original design. The corresponding electrical
demand is 1,269 kWh, reflecting 74% energy reduction compared with the ASHP system. Note that
further energy-savings can be achieved if the collector area is increased; however, the cost will also
increase.
The optimized RVA based on economic performance is considerably lower than those based on
energy performance for large district heating systems (Dahash et al. 2019) but close to those suggested
for small systems (Li et al. 2014; Ma et al. 2018). In a study by Durão et al. (2014) in Portugal, the
Figure 9. Minimum collector areas for given tank volumes to meet the heating demand plotted with the corresponding electrical
demand and the equivalent annual cost.
ENERGY SOURCES, PART B: ECONOMICS, PLANNING, AND POLICY 11
optimal RVA value for the lowest annual cost was calculated to be 4.8 m
3
/m
2
(1630/339) when the
STES met 100% of the heating demand. However, the authors did not provide the market parameters
used in their model. Milewski () recommended an RVA value of 0.7 m
3
/m
2
for a reduced size system
with a solar fraction of 53% after considering the total affordable initial cost rather than considering
energy-saving returns. The smaller optimized RVA value in our study confirms the fact that seasonal
storage is still an expensive technology for small-to-medium-sized STES systems in China.
3.3. Eect of market variables
Three market variables were examined for their influence on the economic performance of the STES:
the tank price, collector price, and utility price. Figures 10 and 11 show the payback periods for
replacing systems B and C with system A at different tank prices. Note that the line of P
E
= 0.538 CNY/
kWh and P
C
= 1000 CNY/m
2
does not interact with the line P
T
= 1867 CNY/m
3
, implying that
replacing system B with system A can never be profitable. This result is expected, considering that the
STES is not even an economical choice compared with a conventional heating source such as natural
gas or electricity (McKenna, Fehrenbach, and Merkel 2019). Here the ASHP is an efficient and mature
market product.
Figures 10 and 11 show that the payback period decreases when the tank price or collector price
decreases or the utility price increases. For the STES system to be profitable in practice, the payback
period should be no more than the lifetime of the system (n
0
< 20 y). The threshold values (at n
0
= 20 y)
of the three market prices are P
E
= 2.5 CNY/y, P
T
= 700 CNY/m
2
, and P
C
= −166 CNY/m
2
.
Independently, the practical applicability of these thresholds does not appear to be realizable soon
for small systems as the values largely deviate from the current market values. The negative threshold
value of the collector price implies that payback for the investment will be impossible, considering
Figure 10. Payback periods of system A replacing system B under different market conditions. Solid lines: P
C
= 1000 CNY/m
2
; Dot
lines: P
C
= 800 CNY/m
2
; Dash dot lines: P
C
= 1.0 CNY/m
2
.
12 J. LU ET AL.
current tank prices and utility energy prices. A combination, however, is more practical. For example,
the payback period can be reduced to 10 y at P
E
= 1.2 CNY/kWh, P
T
= 1100 CNY/m
2
, and P
C
= 800
CNY/m
2
, which is a more practical combination through incentives. Note that the threshold P
T
= 700
CNY/m
2
may be realizable for large storage tanks, as discussed in section 2.3, suggesting that the large-
scale STES system for district heating projects may be already market-ready. This situation confirms
some of the existing studies that predicted the market entry of district-scale STES systems in 2020 (IEA
2015; McKenna, Fehrenbach, and Merkel 2019).
Replacing system C with system A becomes economically advantageous either when the utility price
reaches P
E
= 0.838 CNY/kWh or when the tank price drops below P
T
= 1700 CNY/m
3
. Both values are
practical. In fact, in Hangzhou, the residential utility price is 0.838 CNY/kWh after 4,800 kWh within
the year. For the storage tank, such a price is realizable. Therefore, the current policy supporting SWH
should shift toward supporting the installations of both collectors and the seasonal storage system.
The effect of market variables on the optimal RVA value for best economic performance is shown
in Figure 12. The optimal ratio increases as the tank price decreases, as the utility price decreases, or as
the collector price increases. In building integrated systems, a high RVA value is favorable because
space is limited, and larger storage volume will have a larger solar fraction (Beausoleil-Morrison et al.
2019). The ratio is sensitive to the price of the tank and the collector, and, to a lesser extent, the utility
price. For example, under current market conditions, the ratio would increase to 0.7 m
3
/m
2
if the tank
price declines to 1000 CNY/m
3
. This ratio is in the range of medium-to-large-scale systems suitable for
space heating for a large building complex or a community. Figure 12 can be used as a guideline for the
design of the STES+SWH system in Hangzhou or other cities with similar solar resources.
3.4. Eect of interest rate
The interest rate depends on the country and the availability of incentives. A relatively wide range of
interest rates has been used in the literature. Interest rates from 1.71% in Turkey (Martinopoulos and
Figure 11. Payback periods of system A replacing system C (right) under different market conditions.
ENERGY SOURCES, PART B: ECONOMICS, PLANNING, AND POLICY 13
Tsalikis 2014) to 12% in Egypt or Saudi Arabia (El-Bialy et al. 2016) have been reported. In the EU, 3%
has been used (Hirvonen, Ur Rehman, and Sirén 2018).
In this studied case, the influence of interest rates on the EAC of the three systems is shown in
Figure 13. Although the EAC in general decreases as the interest rate decreases, the decreasing rate is
faster for more expensive systems. With a zero-interest rate, system A has the same annual cost as
system C, indicating that the energy-savings from the seasonal storage system balance the increased
investment cost within the lifetime of the system. The figure also shows that system A still cannot
compete against system B in the market if the discounted interest rate is the only incentive measure.
However, if the construction cost of the tank can be reduced to 800 CNY/m
3
, the EAC of system
A becomes lower than that of system B once the interest rate is less than 3%, confirming the previous
finding that large STES systems may be market competitive.
3.5. Eect of the storage period
For systems with a smaller RVA value, it is tempting to shorten the collecting period as there
is more energy in the non-heating season than can be stored. It was discovered that the solar
collecting period can be reduced without sacrificing the amount of heating energy delivered.
As shown in Figure 14, the best energy efficiency is achieved when the starting date for
charging is postponed until July 17. Similar results can be found in (Li et al. 2015), where the
best starting time was found to be May 1 instead of March 15, the end of the heating season,
for the optimization of STES system. However, in this study, the total energy-savings by
postponing the starting date is small: 45 kWh, or a 2% reduction in the overall energy
demand. The reason is that the pumping energy of the solar loop has a small share in the
total electrical demand.
0
0.3
0.6
0.9
1.2
1.5
0 500 1000 1500 2000 2500 3000
eulavAVRlamitpO
Tank price, CNY/m
3
500 800 1000 1250 1500
500 800 1000 1250 1500
Collector price: CNY/m2
Figure 12. Effects of the tank price, collector price, and utility price on the variation of the optimal RVA value for the STES+SWH
system (system A). Solid lines: P
E
= 0.538 CNY/kWh; Dashed lines: P
E
= 0.838 CNY/kWh.
14 J. LU ET AL.
4. Conclusions
This study evaluated the techno-economics of replacing the ASHP system with the STES system for
residential space heating under the climatic conditions of Hangzhou city in China. Simulation in
TRNSYS showed that the STES with an RVA = 40/40 m
3
/m
2
achieves a solar fraction of 81% and an
6,000
8,000
10,000
12,000
14,000
16,000
18,000
0 0.02 0.04 0.06 0.08
YNC,tsoclaunnA
Interest rate, i
System A
System B
System C
System A
Figure 13. Effect of interest rate on the annual cost of three systems. Solid lines: current market conditions; Dashed lines: current
market condition except for P
T
= 800 CNY/m
3
.
Figure 14. Relationship between the starting date of seasonal storage and the total electricity demand.
ENERGY SOURCES, PART B: ECONOMICS, PLANNING, AND POLICY 15
overall system COP of 3.95. Compared with the ASHP system, the STES achieves 40% energy-saving.
Compared with conventional solar heating system, the STES could increase the energy capacity of
collectors from 51 to 229 kWh/m
2
.
The RVA of the STES system was optimized using the EAC method, and the sensitivity of market
variables was analyzed. The most economic RVA was found to be 0.33 m
3
/m
2
under current market
conditions for the studied case. This RVA corresponds to 66 m
2
of collector area, and 20 m
3
of tank
volume. The electricity demand was 1,269 kWh, reflecting 74% of the energy-saving compared with
the ASHP heating system. Sensitivity analysis showed that the RVA is sensitive to market prices. The
ratio increases when the tank price increases or when the collector price decreases or when the utility
price is higher.
Despite the considerable energy performance of the STES over the ASHP system or conventional
solar-assisted ASHP system, its replacement of the ASHP system for space heating is not economical at
current market prices of electricity, tank storage, and collectors. For the STES to be market compe-
titive, the threshold prices for electricity and the storage tank were calculated to be 2.5 CNY/kWh and
700 CNY/m
3
, respectively. Large STES systems may already be market-ready because of the reduction
of storage tank price. For small systems, policy support is required. Reduction in the collector price or
the interest rate alone cannot achieve a realizable payback period; however, it could play a role in
a combination of policy measures that aim to achieve the above price thresholds. For heating systems
already designed with solar collectors, the addition of STES is economically beneficial considering
current market conditions.
In addition to the effort of reducing unit prices of tanks, collectors, and electricity, reducing the
physical size of the storage system while maintaining the same storage capacity is desirable for
residential application. The breakthrough may lie in new materials and technologies associated with
high energy storage density, such as the adsorption storage technology (Xu and Wang 2019). Finally,
our analysis does not consider the greater demand flexibility offered by the seasonal energy storage. In
the future, the quantification of demand flexibility (Stavrakas and Flamos 2020) and its associated
economic and social benefits could be taken into account.
Funding
The work was supported by the Department of Science and Technology of Zhejiang Province through project
[2014C31015].
ORCID
Guoqing He http://orcid.org/0000-0002-7667-2335
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