For higher solar fraction and suitability for both heating and cooling, a solar heat pump system with seasonal storage was studied in this paper. 
The system scheme and control strategy of a solar heat pump system with seasonal storage for heating and cooling were set up, which is responsible for the space heating and cooling and domestic hot water for a residential block. Through hourly simulation, the performance and the economics of such systems were analyzed, for the different tank volumes, operating modes and weather conditions. 
The results show that 1) for most areas of China, the solar systems with seasonal storage can save energy; 2) for areas with cold winter and hot summer, it is suitable to store heat from summer to winter and store cold energy from winter to summer, but for chilly areas, it is suitable to only store heat from summer to winter; 3) when the ratio of volume of seasonal storage tank to collector areas is 2~3, the system performance is optimal and the payback period is shortest for most areas of north China; and 4) if  cooling storage is needed, the seasonal storage coupled with short-term storage may raise the solar fraction largely

Chen, P.

Dalenback, J.

Yu, G.

Texas A&M University

ICEBO2006, Shenzhen, China                                    Renewable Energy Resources and a Greener Future Vol.VIII-2-2 
 
Preliminary Analysis of a Solar Heat Pump System with Seasonal Storage for 
Heating and Cooling 
 
Guoqing  Yu Peng  Chen Jan-Olof  Dalenback 
Associate Professor Graduate student Professor 
University of Shanghai for Sci. 
and Tech 
University of Shanghai for Sci. 
and Tech 
Chalmers University of 
Technology 
Shanghai & China Shanghai & China Gothenberg & Sweden 
guoqing.yu@163.com 
 
Abstract: For higher solar fraction and suitability for 
both heating and cooling, a solar heat pump system with 
seasonal storage was studied in this paper.  
The system scheme and control strategy of a solar 
heat pump system with seasonal storage for heating and 
cooling were set up, which is responsible for the space 
heating and cooling and domestic hot water for a 
residential block. Through hourly simulation, the 
performance and the economics of such systems were 
analyzed, for the different tank volumes, operating 
modes and weather conditions.  
The results show that 1) for most areas of China, the 
solar systems with seasonal storage can save energy; 2) 
for areas with cold winter and hot summer, it is suitable 
to store heat from summer to winter and store cold 
energy from winter to summer, but for chilly areas, it is 
suitable to only store heat from summer to winter; 3) 
when the ratio of volume of seasonal storage tank to 
collector areas is 2~3, the system performance is 
optimal and the payback period is shortest for most 
areas of north China; and 4) if  cooling storage is needed, 
the seasonal storage coupled with short-term storage 
may raise the solar fraction largely. 
Key words： Solar heating, Seasonal Storage, Heat 
pump 
 
In most area of China, it is cold in winter and hot 
in summer. There are about 2/3 areas with annual 
total horizontal irradiation above 5000MJ/m2 in 
China. The resources of solar energy are abundant 
and they have large potential for solar heating and 
cooling in these areas. 
In order to increase the solar fraction of solar 
heating systems, the concept of seasonal storage was 
put forward and some demonstration projects have 
been built in Europe since 1980’s [1]. Seasonal storage 
charges solar heat collected in summer and discharge 
heat in winter. To increase the available temperature 
differences of the seasonal storage and hence reduce 
the storage size, some solar heating projects  
introduces heat pumps  
 
1. INTRODUCTION TO THE SOLAR HEAT 
PUMP SYSTEM WITH SEASONAL 
STORAGE 
 
1.1 Schematic Diagram and Operating Strategies of 
Solar Heat Pump System with Seasonal Storage 
In order to meet the requirement of heating in 
winter and cooling in summer, the schematic of solar 
heat pump system with seasonal storage was put 
forward, as in Fig. 1[2].  
The system includes collector arrays (A), a 
seasonal storage water tank (B), a domestic hot water 
tank (D), heat pumps (E), boilers(F,H), cooling 
towers(G), and some pumps. 
In this system, it uses Domestic Hot Water (DHW) 
priority strategy, that is, the collected solar energy is 
stored in the storage first, after its temperature is 
greater than the set temperature (such as 60℃), the 
solar energy is stored in the seasonal storage. 
The space heating and cooling operation model 
during a whole year are as below: 
(1) Operation mode in winter 
If the temperature in the seasonal storage is 
greater than the heating supply temperature (such as 
60℃), the water in the seasonal storage is used to 
meet the heating demand directly. When the 
ESL-IC-06-11-261 
Proceedings of the Sixth International Conference for Enhanced Building Operations, Shenzhen, China, November 6 - 9, 2006 
ICEBO2006, Shenzhen, China                                    Renewable Energy Resources and a Greener Future Vol.VIII-2-2 
temperature is between 50 ℃  and 60 ℃ , the 
temperature of the supply water from seasonal tank is 
raised to 60℃ by boiler and then sent to the load. 
When the tank water temperature is 10℃ to 50℃, the 
heat pumps extracts heat from the seasonal storage 
and heat the return water form the load side. If the 
temperature is below 10℃ , heat pumps stops and 
seasonal storage are bypassed, while boilers are used 
to direct heating. Since then the seasonal storage was 
used to store cold energy for summer cooling. 
(2) Operation mode in spring 
The seasonal storage is used to store cooling 
energy. The tank water temperature increases 
gradually because of heat gains from ambient. The 
collected solar energy only stored in the DHW tank.  
(3) Operation model in summer 
In this season, the cooling demand is met by the 
heat pump or the seasonal storage. When the 
temperature in the seasonal storage is below 15℃, 
the water in it is supplied as chilled water to the load. 
When the temperature is 15℃ to 40℃, the heat pump 
begin to work as chillier and seasonal tank water is  
 
L
5A
B
2a
1a
2b
6
J
E
H
t o 
user
D F
K
DHW
3 4 7
11
10
G
wat er  
suppl y
f r om 
user8 9
1bI
 
Fig. 1 Schematic of solar heat pump systems with seasonal storage 
used to cool the condenser of the heat pumps. If the 
temperature is greater than 40℃, the cooling tower is 
started and the seasonal storage begin to store solar 
energy for winter heating. 
(4) Operation model in autumn 
The task of this season is to discharge as much 
heat as possible into the seasonal tank for winter use. 
1.2 Overview of the Virtual Case 
In order to study the characteristics of such 
systems and the feasibility of applying them to China,  
a virtual residential district in the suburb of  Beijing 
was assumed, then a solar heat pump system with 
seasonal storage was designed. This system is 
responsible for the heating and cooling of the virtual 
residential district of floor area of 23000m2. The 
design space heating load is 1124kW, design space 
cooling load is 390kW, 60oC DHW 60m3 per day. 
The collectors is stalled on the building roof and 
inclined 30°to the horizontal plane, facing north. 
The total collector area is 2900m2 or so. 
The seasonal storage is hot water storage and 
stands above ground. The ratio of volume of seasonal 
tank to the collector area (it will be reduced to Rv in 
the following sections) is 2. That means the volume 
of seasonal storage tank is 5800m3. 
After all main features of this system was 
determined, some computer routines were made, and 
many simulations were conducted hour by hour for 
many different parameters and conditions based on 
 
ESL-IC-06-11-261 
Proceedings of the Sixth International Conference for Enhanced Building Operations, Shenzhen, China, November 6 - 9, 2006 
ICEBO2006, Shenzhen, China                                    Renewable Energy Resources and a Greener Future Vol.VIII-2-2 
TMY weather data within the life cycle. Some 
important results will be discussed in the following 
sections. 
2. ANALYSES OF THE BASE CASE 
For the DHW load, from the Fig. 2, we can see 
that the domestic hot water load can be met mostly by 
solar energy and the solar fraction achieves 87.5% in 
a whole year. 
For the space heating load, the thermal energy in 
the seasonal storage tank can be extracted directly or 
by heat pump to meet the load. The total thermal 
energy stored by the seasonal storage can meet the 
total space heating load about 17.1%, as in Table 1. 
For space cooling load, the water in the seasonal 
storage tank can be used as direct cooling when its 
temperature is below 15℃ . If the temperature is 
higher, it can be used as the condenser cooling water 
of heat pumps, it’s useful for reducing the energy 
consumption of the cooling tower and charging heat 
into seasonal tank quickly. The total cooling load met 
by seasonal storage tank is about 29.7% directly or 
indirectly as in Table 2. If the terminals of air 
conditioning system can tolerate higher chilled water 
temperature, it will be able to use more free cooling. 
 
 
Tab.1 Space heating load met by storage   (GJ) 
Month Total heating load 
Direct 
heating  
Heating by 
heat pump 
1 3550 0 0 
2 2710 0 0 
3 1170 0 0 
11 1820 1000 660 
12 3130 0 460 
0
100
200
300
400
500
600
700
800
900
1000
1 2 3 4 5 6 7 8 9 10 11 12
month
So
la
r 
En
er
gy
(G
J)
For DHW For heating  
Fig.2 Use of collected solar heat 
0
20
40
60
80
100
0 1 2 3 4 5 6 7 8 9 10 11 12
month
℃
0.5 1 2 3 4 5  
Fig. 4. Yearly temperature variety for different volume of tank 
 
ESL-IC-06-11-261 
Proceedings of the Sixth International Conference for Enhanced Building Operations, Shenzhen, China, November 6 - 9, 2006 
ICEBO2006, Shenzhen, China                                    Renewable Energy Resources and a Greener Future Vol.VIII-2-2 
Tab. 2 Cooling load  meet by storage tank (GJ) 
Month Total cooling  
Direct 
cooling 
Condenser 
cooling 
6 1550 200 270 
7 1840 0 0 
8 1270 0 0 
9 130 0 0 
 
3. EFFECTS OF SEASONAL STORAGE 
CAPACITY  
The volume of seasonal storage is an important 
parameter of the solar system. This paper use the 
ratio of storage volume to collector area (Rv) to study 
the effects on the whole system. 
Fig. 4 shows the yearly temperature variety for 
different volume of tank. The Rv is from 0.5 to 5. 
From the Fig.4 we can see that the temperature in the 
tank rise to high level in short period when Rv is 
smaller than 2. If the Rv is more than 3, the 
temperature in the tank rises slowly. When the winter 
coming, the temperature is still in low level and the 
available temperature difference of storage is small. 
The capability of storage tank is not utilized 
efficiently. The optimal Rv is between 2 to 3, the 
temperature rises to the highest level just at the 
beginning of winter. Furthermore, the yearly collector 
efficiency is also higher (shown in Fig. 5). 
The larger the volume of the tank, the more the 
solar energy can be collected (shown in Fig. 6). For 
the bigger seasonal tank, the energy supplied by the 
auxiliary boiler is less and the energy saved is more. 
However, Fig. 5 indicates that when the Rv is more 
than 3, the annual collector efficiency begins to 
reduce. 
From the above discussion, Rv between 2 and 3 is 
an optimal value for this system. This conclusion 
agrees with the European experience of solar heating 
system with seasonal storage[3]. 
0%
10%
20%
30%
40%
0.5 1 2 3 4 5
Heating Solar Fraction Yearly Collector Efficiency  
Fig. 5  Solar Fraction and Collector Efficiency 
0%
20%
40%
60%
80%
100%
0.5 1 2 3 4 5
Auxiliary Boiler Seasonal Storage Tank Heat Pump Power  
Fig. 7. The Composing of Heating Energy 
 
4. EFFECTS OF OPERATING MODE 
From the simulation, it was found that some solar 
heat can not be collected in spring because seasonal 
heat storage were used to store cold energy during 
that period. What will happen if the seasonal storage 
was only used to store heat and not store cold energy? 
This paper defines the operating mode mentioned 
in section 1.1 as Mode 1, and Mode 2 refer to only 
storing heat and no storing cold energy. 
The results of changing from Mode 1 to Mode 2 
are listed in Table 3. 
Tab. 3 Changes from Mode 1 to Mode 2 
City 
Heating 
Increme
nt (GJ) 
Heating 
fraction 
 
Cooling  
reductio
n (109kJ) 
Cooling 
fraction 
reduction 
Beijing +832 +6.7% -202 -12.75% 
Shanghai +90 +2.6% -153 -7.92% 
Harbin +2050 +9.3% -222 -55.84% 
Lanzhou +830 +6.4% -205 -37.55% 
Lhasa +3440 +23.4% -9 -100% 
 
Because solar space heating fraction is low ( such 
as 18% for Bejing case), so this effect is very 
significant. As to Bejing, Harbin, Lanzhou and Lhasa, 
the improvement in heating in much greater than the 
reduction in cooling, but Shanghai is not the case. In 
other words, shanghai would be better to adopt mode 
1 and the other four would be better to adopt mode 2.  
5 IMPROVEMENT BY DIURNAL 
STORAGE 
Another alternative is to add a diurnal heat storage 
tank which is parallel to the seasonal tank. When the 
seasonal storage begin to store cooling energy in the 
end of space heating season. The solar heat collected 
ESL-IC-06-11-261 
Proceedings of the Sixth International Conference for Enhanced Building Operations, Shenzhen, China, November 6 - 9, 2006 
ICEBO2006, Shenzhen, China                                    Renewable Energy Resources and a Greener Future Vol.VIII-2-2 
 
will be charged in the diurnal storage and used for 
space heating. This mode is defined as Mode 3. 
The Changes from Mode 1 to Mode 3 of Beijing 
base case are listed in Table 4. It shows that the solar 
space fraction is also significant, and the 
improvement is near to Mode 2. It also was found 
that a diurnal storage of 75-150L per collector area is 
enough. This agrees with the European experience of 
solar heating system with diurnal storage[4]. 
Tab. 4 Changes from Mode 1 to Mode 2 
 Diurnal 
storage 
(m3) 
Diurnal 
storage per 
collector 
area (L/ 
m2) 
Heating 
increment 
(GJ) 
Solar space 
heating 
fraction 
increment 
500 172.7  +859 +6.92% 
434 149.9  +847 +6.82% 
289.5 100.0  +803 +6.47% 
217 75.0  +760 +6.13% 
108.5 37.5 +588 +4.74% 
6. CONCLUSIONS 
From the simulation and analysis, it shows that:  
(1) For most area of China, the solar systems with 
seasonal storage can save energy;  
(2) For areas with cold winter and hot summer, it 
is suitable to stores heat from summer to winter and 
store cold energy from winter to summer, however 
for chilly areas, it is suitable to only store heat from 
summer to winter; 
(3) If the cooling storage is needed, the seasonal 
storage coupled with short-term storage can raise the 
solar fraction significantly.  
(5) When the ratio of volume of seasonal storage 
tank to collector areas is 2~3, the system performance 
is optimal for many areas of China. 
 
REFERENCE 
[1] Jan-Olof Dalenback, Central solar heating plant with 
seasonal storage---Status Report[M], IEA Technical 
Report, 1990.  
[2] Peng Chen. Preliminary Study of Solar Heating and 
cooling Systems with Seasonal Storage[D]. 
Shanghai: University of Shanghai for Science and 
Technology, 2006. 
[3] T.Schmidt, et al. Central solar heating plants with 
seasonal storage in Germany[J].  Solar energy. 
2004，76 : 165-174.  
[4] Werner Weiss. Solar heating systems for houses[M]. 
Jame & James Ltd. 2003. 
 
 
 
 
ESL-IC-06-11-261 
Proceedings of the Sixth International Conference for Enhanced Building Operations, Shenzhen, China, November 6 - 9, 2006 


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Preliminary Analysis of a Solar Heat Pump System with Seasonal Storage for Heating and Cooling

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Preliminary Analysis of a Solar Heat Pump System with Seasonal Storage for Heating and Cooling

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