Sizing of Electrical and Thermal Storage Systems in the Nearly Zero Energy Building Environment – A Comparative Assessment

Abstract

In order to determine the most suitable storage system for buildings with photovoltaic generators (PVs), a holistic analysis of the energy consumption is often required. In that sense, this paper provides a comparative assessment of thermal and battery storage in the context of nearly zero energy buildings (NZEBs). A NZEB with a PV, heat pump and a radiant floor has been considered for this purpose. The comparison is conducted in terms of the self-consumption rate (SCR) and the net present value (NPV). In addition, a control strategy that maximizes the buildings self-consumption has been developed for the NZEB with thermal storage. For the NZEB with a battery, a common rule-based strategy has been employed. The results indicate that for smaller capacities, the thermal storage is better at improving self consumption. Moreover, under a pure self-consumption scheme, battery storage becomes economically viable if investment costs are lower than 200 EUR/kWh.

Full text

Sizing of Electrical and Thermal Storage Systems
in the Nearly Zero Energy Building Environment –
A Comparative Assessment
Vladimir Gjorgievski, Angelos I. Nousdilis, Eleftherios O. Kontis,
Georgios C. Kryonidis, Georgios A. Barzegkar-Ntovom, Snezana Cundeva,
Georgios C. Christoforidis, Grigoris K. Papagiannis
Abstract—In order to determine the most suitable storage
system for buildings with photovoltaics (PVs), a holistic analysis
of the energy consumption is often required. In that sense, this
paper provides a comparative assessment of thermal and battery
storage in the context of nearly zero energy buildings (NZEBs).
A NZEB with a PV, heat pump and a radiant floor has been
considered for this purpose. The comparison is conducted in
terms of the self-consumption rate and the net present value. In
addition, a control strategy that maximizes the buildings self-
consumption has been developed for the NZEB with thermal
storage. For the NZEB with a battery, a common rule-based
strategy has been employed. The results indicate that for smaller
capacities, the thermal storage is better at improving self-
consumption. Moreover, under a pure self-consumption scheme,
battery storage becomes economically viable if investment costs
are lower than 200 EUR/kWh.
Index Terms—Battery storage, building model, photovoltaics,
self-consumption, thermal storage.
I. INTRODUCTION
The European Union aims to improve energy efficiency
by 32.5% until 2030 [1]. This calls for significant action,
especially in the building sector, as it accounts for about
40% of the overall energy consumption. In order to meet this
goal, existing buildings should be transformed into nearly zero
energy buildings (NZEBs) by reducing their energy intensity
and by integrating renewable energy sources to meet their local
demand [2]. Nevertheless, as climate conditions and building
stock characteristics vary geographically, it is up to Member
States to provide country specific NZEB definitions [3].
Vladimir Gjorgievski and Snezana Cundeva are with Faculty of Electrical
Engineering and Information Technologies, University Ss Cyril and Method-
ius, Skopje, Republic of North Macedonia.
Angelos I. Nousdilis, Eleftherios O. Kontis, Georgios C. Kryonidis and
Grigoris K. Papagiannis are with Power Systems Laboratory, School of
Electrical and Computer Engineering, Aristotle University of Thessaloniki,
Thessaloniki, Greece.
Georgios A. Barzegkar-Ntovom is with Power Systems Laboratory, De-
partment of Electrical and Computer Engineering, Democritus University of
Thrace, Xanthi, Greece.
Georgios C. Christoforidis is with Western Macedonia University of Ap-
plied Sciences, Kozani, Greece
This work has been co-funded by the European Union and National Funds
of the participating countries through the Interreg-MED Programme, under
the project ”PV-ESTIA - Enhancing Storage Integration in Buildings with
Photovoltaics”.
In countries with high solar irradiation, for example, it may
be economically viable to install rooftop photovoltaic (PV)
generators [4]. The profitability of such systems often depends
on the inherent self-consumption of the building [5]. One
of the ways to achieve higher self-consumption rate (SCR)
is by electrifying the building heating system [6]. However,
additional measures are necessary to significantly reduce the
energy flows between the building and the grid. This can
be achieved by utilizing the storage capabilities of buildings,
either in the form of deferrable end use appliances [7], building
thermal inertia [8] or by integrating a battery [9] or thermal
storage system [10]. There have been numerous publications
dealing with battery and thermal energy storage systems in
buildings [11]. However, the number of papers that provide
a comparison of these storage alternatives, such as [12], are
limited.
This paper aims to investigate the energy performance
of a NZEB with a PV system under the operation of two
distinct energy storage technologies - an electrical and a
thermal storage. Simple rule-based control strategies are used
to simulate their operation. Because of the intrinsically dif-
ferent technical capabilities of the storage technologies, the
comparison is conducted in terms of the SCR and the net
present value (NPV).
II. BUILDING MODEL
It has been assumed that building heating and cooling
demand are met by an air-to-water heat pump connected to a
radiant heating system, while a rooftop PV generator supplies
part of the electricity demand. To increase its flexibility,
as shown in Fig. 1, the building is equipped either with a
hot water tank (thermal storage) or a battery storage system
(electrical storage). The building is modelled with the dynamic
building model presented in [13].
A. Thermal power
The thermal power demand of the building Pth is calculated
using the following equation:
Pth(t)=Pth,des (t)1−
¯
Tex(t)−Tdes
Toff −Tdes (1)
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Citation Information: DOI: 10.1109/SyNERGY-MED.2019.8764142

where Pth,des(t)denotes the thermal power required by the
building when the effective outside temperature ¯
Tex(t)is equal
to the design temperature Tdes.Tof f represents the tempera-
ture at which the heating/cooling is switched off. When ¯
Tex(t)
is lower than a certain threshold Th
of f , heating is required and
the heating system turns on. Thus, Toff =Th
off is substituted
in the equation (1). Similarly, if ¯
Tex(t)≥Tc
of f , cooling is
required and hence, Toff =Tc
of f . Suitable values are also
substituted for the design temperature Tdes when determining
heating demand (Tdes =Th
des) and cooling demand (Tdes =
Tc
des). The effective outside temperature ¯
Tex(t)is calculated
as a moving average of the previous ¯
φvalues of the external
temperature Text(t)allowing the building thermal dynamics
to be taken into account. The parameter ¯
φis determined by
(2) and denotes the building’s time shift.
¯
φ=N
i=1 (UA)iφi
N
i=1 (UA)iφi+Hve
(2)
In (2) (UA)iis the thermal transmittance of the i-th surface
of the building envelope, φidenotes the characteristic time
shift of each surface, while Hve is the equivalent ventilation-
thermal transmittance. A time shift φrepresents the time delay
between the maximum of a cause and the maximum of its
effect. Note that, Nis the total number of building surfaces.
B. Domestic hot water (DHW)
The power required for domestic hot water preparation can
be calculated with the following equation:
Pdhw(t)= ˙mwcw(Tw,out(t)−Tw ,in(t)) (3)
where ˙mwis the mass flow of the consumed hot water, cw
is the specific heat capacity of water, while Tw,out(t)and
Tw,in(t)are the temperatures of the domestic hot water and
cold water coming from the mains, respectively.
C. Radiant floor
The temperature of the radiant floor Trf (t)at time step tis
calculated as:
Trf (t)=Tw,rf (t)−Pth(t)
Srf Urf
(4)
where Tw,rf (t)is the temperature of the water contained
in the radiant floor pipes, Srf is the surface of the radiant
floor heating and Urf represents its equivalent transmittance.
The water temperature can be calculated using the following
equation:
Tw,rf (t)=Tin(t)+ΔTrf,rated Pth(t)
Srf Krf,rated
1
nrf (5)
where Tin(t)is the desired room temperature inside the build-
ing, ΔTrf,rated is the rated temperature difference between
the circulating water and the inside ambience, Krf,rated is
the rated power output of the radiant floor per surface unit
and nrf is the emitter exponent of the system. It has been
mÉä
méî
mÉä
méî
mÜé
mÉÜ
qÅçåÇ=
qï IêÑ
qíë=
qï Içìí
qï Iáå
mÇÜï qêÑ
mÜé
qÅçåÇ=
qêÑ=
qï IêÑ
mÇÜï
mÖêáÇ
qÉî~ =míÜ
=
qÉî~ =
míÜ
=
mÖêáÇ
mÄ~í
~F
ÄF
mÜéIíÜ
mÜéIíÜ
Fig. 1. System configuration a) NZEB with thermal storage and b) NZEB
with battery storage.
assumed that the room temperature is constant and equal to
a set-point temperature (20oC when heating and 25oC when
cooling).
D. Heat pump (HP)
The thermal power output of the heat pump Php,th(t)at time
step tis a product of the heat pump electric power Php(t)and
its coefficient of performance COP(t)or energy efficiency
ratio EER(t), depending on the operating mode. Hence, the
thermal power of the heat pump in heating mode is equal to:
Php,th(t)=Php (t)COP(t)(6)
while in cooling mode it is equal to:
Php,th(t)=Php (t)EER(t)(7)
The COP(t)and EER(t)are calculated as the product of the
heat pumps efficiency ηhp and Carnot’s efficiency, represented
as a function of the evaporator Teva(t)and condenser Tcond(t)
temperatures.

COP(t)=ηhp
Tcond(t)
Tcond(t)−Teva (t)(8)
EER(t)=ηhp
Teva(t)
Tcond(t)−Teva (t)(9)
Temperature drops of 5 K and 10 K are assumed to occur
at the indoor and outdoor heat exchangers of the heat pump,
respectively.
E. Thermal storage system (TSS)
The thermal storage serves the heating demand and domes-
tic hot water consumption of the building. It is heated by a heat
pump and an electric heater (EH). It has been assumed that the
temperature field inside the thermal storage is homogeneous.
Hence, the one node model of a thermal storage is used [14]
and the temperature of the thermal storage Tts(t)is determined
by a discrete form of the dynamic energy balance equation:
ρwcwVts (Tts(t)−Tts (t−1)) = Pk(t)Δt(10)
where ρwand cware the density and specific heat capacity of
water, Vts is the volume of the storage tank and Δtdenotes
the duration of one time step. The sum Pk(t)represents
the thermal power balance of the storage:
Pk(t)=Php,th(t)+Peh (t)−Pth(t)−Pdhw (t)
−Sts
λts
dts
(Tts(t)−Tex (t)) (11)
Equation (11) takes into account the power supplied to the
thermal storage by the heat pump Php,th(t)(calculated by
(6)) and the electric water heater Peh(t), as well as the power
supplied by the thermal storage for the heating demand Pth(t),
the hot water demand Pdhw(t)and the heat losses. The heat
losses are given by the last term in (11), where Sts,λts
and dts denote the total surface, the thermal conductivity and
the thickness of the thermal storage tank, respectively. It is
assumed that the tank is located in a poorly isolated area of
the building and that the ambient temperature surrounding the
thermal storage is equal to the external temperature.
F. Battery storage system (BSS)
At each time step, the energy stored in the battery Ebat(t)
is calculated as the sum of the energy stored at the previous
time step Ebat(t−1) and the change in energy due to charging
or discharging:
Ebat(t)=Ebat (t−1) −kPbat(t)Δt(12)
In the equation above Pbat(t)denotes the battery power. It
is negative when the battery is charging and positive when
the battery is discharging. Moreover, kdenotes the losses
occurring from charging and discharging. It is equal to 1/ηdis
when the battery is discharging and equal to ηch when the
battery is charging. Note that, ηch and ηdis are the charging
and discharging efficiency of the battery, respectively.
III. CONTROL STRATEGIES
A. NZEB with thermal storage
A rule-based control algorithm has been developed for a
NZEB with thermal storage. The control algorithm, depicted in
Fig. 2, aims to improve building self-consumption by utilizing
the surplus PV generation. When ¯
Tex(t)≤Th
off and the
building requires space heating, the heat pump is used to
convert surplus PV power to heat. The control algorithm
employed in this case is shown on the left side of the
diagram in Fig. 2. The surplus energy is used to increase the
temperature of the thermal storage and can be later used to
cover the space heating Pth(t)and hot water demand Pdhw(t).
In case Tts(t)exceeds the maximum temperature Tup , the heat
pump is turned off and Pth(t)and Pdhw (t)are supplied only
by the thermal storage. On the other hand, when Tts(t)falls
below the minimum acceptable temperature Tdown, the heat
pump operates at its rated capacity Php,rated and an additional
thermostatically controlled electric heater is turned on.
When the building has no heating demand, the surplus PV is
captured by the electric heater. During this time, the heat pump
is either turned off or operates in cooling mode, i.e. bypasses
the hot water tank and directly feeds the radiant floor. Hence,
the water in the tank is heated only by the electric heater.
In order to utilize the surplus PV generation most effectively,
instead of a simple on/off thermostatic control, a modulating
control has been imposed on the electric heater. When the
temperature of the thermal storage falls below Tdown, the
thermostatic control still applies and the electric heater is
turned on, heating thermal storage with a power of Peh,rated.
The control algorithm used for this purpose is shown on the
right side of Fig. 2.
B. NZEB with battery storage
Similarly, a simple rule-based strategy aiming to maximize
building self-consumption is also used to control the battery.
The battery starts charging as soon as there is surplus en-
ergy generated by the PV and discharges when the building
consumption exceeds PV generation. At each time step, the
algorithm simulating the battery’s operation takes into account
the stored energy at the previous time step and checks whether
charging/discharging would violate the battery capacity and
power constraints.
IV. CASE STUDY
A single house with a total floor area of 150 m2and a PV
generator with installed capacity of 6.105 kW was considered
in the analyses. The input parameters for the building model
are shown in Table I. The hourly diagrams for the PV
generation Ppv and electricity consumption Pel were obtained
from [15] and correspond to household number 93 of the
database. The electricity consumption was initially obtained in
a disaggregated form, separately for each appliance. Hence, the
electricity consumption corresponding to the air conditioner
was eliminated and Pel was calculated as the sum of the
electricity consumption of all other appliances. Consequently,
the heating/cooling system can be separately modelled. It was

TtsEtJNF[Tup=
PhpEtF=Z=M Tdown YTts EtJNF≤Tup=
PpvEtF[PelEtF= TtsEtJNF≤Tdown =
Ptarget EtF=Z=PpvEtF=J=PelEtF=
PhpEíF=Z=ãáåôPtargetEtFI=Php,rated õ= Php EtF=Z=M Php EtF=ZPhp,rated =
Peh EtF=ZPeh,rated =
vÉ ë =
vÉ ë =
vÉ ë =
kç =
kç =
bî~äì~íÉ=Tex EtF
TtsEtJNF[Tup=
Peh EtF=Z=M Tdown YTtsEtJNF≤Tup=
PpvEtF[PelEtFHPhpEtF= Tts EtJNF≤Tdown =
Ptarget EtF=Z=Ppv EtF=J=PelEtF=JPhp EtF=
Peh EíF=Z=ãáåôPtarget EtFI=Peh ,rated õ= Peh EtF=Z=M Peh EtF=ZPeh,rated
vÉ ë =
vÉ ë =
vÉ ë =
kç =
kç =
kç = kç =
eÉ~íáåÖ=êÉèìáêÉÇ\
vÉ ë = kç =
Fig. 2. Control algorithm for NZEB wih thermal storage.
also assumed that the building is equipped with a DAIKIN
air-to-water heat pump [16]. The daily hot water consumption
profile was generated using the standard profiles given in
[17]. The hot water consumption for the whole year was then
calculated by multiplying the daily profile by a random number
between 0.8 and 1.2 to avoid an identical daily profile. Then,
it was adjusted so that the associated energy use amounts to
approximately 5 kWh per day, assuming a 45oC temperature
difference (Tw,out =55
oC, Tw,in =10
oC), according
to EN 16147:2011 [17]. The ambient temperature data was
obtained from Meteonorm [18]. Interpolation was used to
convert hourly data into time series with a 15 min time step.
For the economic analysis, the NPV was calculated based
on the methodology provided in [19]. The electricity prices
TABLE I
INPUT DATA FOR THE BUILDING MODEL
Element Data
Building cell A= 150 m2;U=0.3W
m2K;φ=8h;
Pth,des =5kW; Hve =0 W
m2K;Th
des =−3oC
Tc
des =47
oC; Th
off =17
oC; Tc
off =26
oC;
Radiant floor ΔTrf,rated =20
oC;Srf =80m2;nrf =1.1;
Krf,rated =60 W
m2;U=6 W
m2K;
Heat pump heating: Prated
hp,e =2.41 kW; ηhp =0.45;
cooling: Prated
hp,e =3.05 kW; ηhp =0.35;
COP =4.66; EER =3.98;
Thermal storage Vts = [150,600]l; λts =0.04 W
mK ;dts =0.08 m;
system Tup =80
oC; Tdown =40
oC; Tset =50
oC;
Battery storage Emax
bat =[1,10] kWh; Emin
bat =0kWh;
system C-Rate = 0.5; ηbat =0.92;
used in the analyses are provided in Table II.
TABLE II
ELECTRICITY PRICES
Type of cost Amount (cEUR/kWh)
Netted Cost 11.58
Grid Demand Cost 5.82
Services of General Interest Charge 0
Fixed Cost 35.20
For the NZEB with thermal storage, tank volumes in
the range of Vts = [150,600] litres were examined.
It was assumed that the investment cost of the TSS is
1 EUR/litre. Similarly, battery storage capacities in the range
of Emax
bat =[1,10] kWh were considered for the alternative
NZEB scenario. Investment cost of 200 EUR/kWh for the
battery and 350 EUR/kW for the inverter were assumed for this
case. A pure self-consumption scheme where the consumer is
given no reimbursement for the energy he supplies to the grid
was considered.
V. R ESULTS AND DISCUSSION
When the NZEB is equipped with a thermal storage, the
stored surplus PV generation can only be used to cover the
heating and hot water consumption. When a battery is installed
instead, the captured surplus energy can be later be used to
cover all electric loads, including the heating and hot water
consumption. In this sense, the thermal storage is more limited
in the flexibility it provides to the building. Nevertheless, it is
able to store large amounts of energy (a 150 litres TSS is
equivalent to about 7 kWh) and can thus result in comparable
SCRs with the ones obtained when a battery is installed. The
SCR is especially important when a pure self-consumption

scheme is examined since it has dominant influence on the
cost-effectiveness of the system.
Fig. 3 shows the daily consumption and generation profiles
of the NZEB with thermal storage and battery for a typical day
in May. It should be noted that when a thermal storage is con-
sidered, the heat pump should operate at higher temperatures
compared to the case where it only supplies the radiant floor.
Consequently, the heat pumps has a lower COP and higher
electricity consumption when directly coupled with a thermal
storage. However, because of the flexibility provided by the
thermal storage, the heat pump can be modulated to utilize the
surplus PV generation in an efficient way. On the other hand,
when a battery is installed, the heat pump is driven only by
the demand of the radiant floor and thus uses less electricity.
The reason for this are the lower temperatures of the radiant
floor (20oC-34
oC) which result in higher COPs.
As shown in Fig. 4, the proposed control scheme for the
NZEB with a thermal storage is quite successful in obtaining a
high SCR. For smaller storage capacities, in particular, thermal
storage results in higher SCRs than when a battery is installed.
For the conducted analyses, it was found that when the hot
water tank is larger than 480 litres and battery capacity exceeds
7.8 kWh, the battery becomes more effective in improving
the self-consumption of the building. This is a result of the
battery’s ability to cover all types of electric loads. Even for
smaller batteries, the building has no grid interaction during
the majority of the evening hours, which cannot be the case
MMWMM MSWMM NOWMM NUWMM OQWMM
JQ
JO
M
O
Q
S
ms
dêáÇ
em
bi
be
~F
MMWMM MSWMM NOWMM NUWMM OQWMM
JQ
JO
M
O
Q
S
ms
dêáÇ
em
biHaet
_^ q
ÄF
mçïÉê=EâtF
mçïÉê=EâtF
Fig. 3. Daily generation and consumption profiles for a typical day in May
of an NZEB with a) thermal storage (Vts = 300 l) and b) battery (Ebat =6
kWh).
OMM PMM QMM RMM SMM
SM
SR
TM
TR
UM
qpp=îçäìãÉ=EäáíêÉëF
p`o=EBF
O Q S U NM
QM
SM
UM
NMM
_pp=Å~é~Åáíó=EâtÜF
p`o=EBF
a)
ÄF
Fig. 4. SCR of the NZEB for different a) TSS and b) BSS capacities.
for the thermal storage, as depicted in Fig. 3. It is evident
that the comparison of the thermal and battery storage system
purely based on their technical performance is rather limited.
Moreover, the prosumer’s choice of a storage system and its
size will likely be governed by the system’s economic per-
formance. Hence, numerous calculations have been conducted
to assess the economic viability of an investment in a storage
system, assuming an existing PV installation.
A battery is not yet viable considering the lowest current
market prices for residential batteries of 600 EUR/kWh [19].
This is the reason behind the investment cost assumption in
the previous section. A break-even point, for which identical
NPVs were obtained for both systems, was achieved when
the battery has a capacity of 4.2 kWh and the volume of the
thermal storage is 310 litres. An investment in both storage
systems in this case, however, was found not to be profitable.
It should be noted that these NPV values are obtained in the
context of the assumed pure self-consumption scheme. The
economic viability of the system will change as the price for
the electricity fed into the grid increases. Nevertheless, the
results show that even under the worst case circumstances,
coupling a heat pump with a thermal storage and using a
modulating control of the electric heater may result in positive
NPVs for higher TSS capacities. On the other hand, in order
for batteries to become viable in this type of a scenario, the
associated investment costs need to be equal to or lower than
200 EUR/kWh.

OMM PMM QMM RMM SMM
JOMMM
JNMMM
M
NMMM
qpp=Å~é~Åáíó=EäáíêÉëF
kms =EbroF
O Q S U NM
JQMMM
JOMMM
M
OMMM
QMMM
_pp=Å~é~Åáíó=EâtÜF
kms =EbroF
~F
ÄF
Fig. 5. Economic performance of the system for different storage capacities.
VI. CONCLUSION
As prices of PVs continue to decline, an increase of
rooftop PV installations on NZEBs is expected. However, the
mismatch between the generation and consumption of such
buildings can reduce their economic viability. This issue has
been addressed by considering the integration of a storage
system in a NZEB. A NZEB with a PV, heat pump and a
radiant floor was analysed in which either a thermal or a
battery storage is integrated. A parametric analysis has been
performed to investigate how the capacity of each storage
system affects the techno-economic performance of the NZEB.
Furthermore, a sensitivity analysis has been conducted to
determine the intersection point of storage capacities that result
in the same NPV.
In the case of a NZEB with thermal storage, the excess PV
generation was utilized in the form of hot water that could
later be used for heating and domestic hot water. Coupling a
thermal storage tank of volumes between 150 and 480 litres
with the heat pump under the proposed control scheme has a
better ability to improve the NZEB’s SCR than batteries with
capacities of up to 7.8 kWh. Assuming Greek retail electricity
prices and no reimbursement for the surplus PV generation,
the intersection point of storage capacities that result in the
same NPV was found when a 310 litre hot water tank and a
4.2 kWh battery were used. These results were obtained for a
200 EUR/kWh investment costs for the battery. It was found
that under this pure self-consumption scheme more expensive
batteries were not economically viable.
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