Ground source heat pump, air recovery source heat pump,
COP, SPF, primary energy, CO2 emission
Andrzej BUGAJ, Maciej MINIEWICZ*
HEAT PUMP SYSTEMS USED FOR HEATING AND COOLING
The paper presents the study on an application of heat pumps in HVAC systems. Different types
of heat pumps are evaluated in terms of covering heating and cooling needs of the building to cover
heating needs of the building. The performance of heat pumps is analyzed in the range of the whole
heating and cooling season with taking into account actual heating and cooling load profiles. The operation of chosen heat pumps in particular HVAC system is described by coefficients of performance
and seasonal performance factors as well as the usage of primary energy. Also, ecological characteristics of heat pumps’ operation are being shortly assessed. The study shows that the renewable energy
system in the form of heat pumps can satisfactory replace the conventional heat production based on
fossil fuels but the environmental impact of heat pump application is not always favorable one.
1. INTRODUCTION
Environmental concern about global warming and local atmospheric pollution is
the primary impetus for various clean energy technologies. For the purpose to avoid
the direct usage of fossil fuels it has been decided to study the application of heat
pumps within heating, ventilating and air conditioning (HVAC) systems. The best
object to demonstrate the benefits of heat pumps’ applications seems to be an educational institution building because then a wide public can be effectively taught about
an attractiveness of clean energy. Therefore, a school building has been chosen to
study a feasibility of heat pumps’ applications. The heat pump concept is wellestablished technology and for its technical solutions I is widely covered in the textbook literature. Also, the specific concept of ground source heat pump (GSHP) has
gained quite considerable popularity in public but most GSHP applications have been
focused on small residential buildings. One can notice that the technical, economic
and ecological aspects of those small installations are not necessarily well applying to
medium and big size buildings, e.g. schools. Some distinguished examples of bigger
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* Instytut Klimatyzacji i Ogrzewnictwa Politechniki Wrocławskiej
heat pump installations can also be found in literature [1], [2], [3], [4]. Those literature
examples show that the big size applications of heat pump can give quite promising
results and as the renewable energy installations can be the only heating source for the
building without any need for an auxiliary heating.
The technical and economic performance of heat pump is closely related to characteristics of the heat source. An ideal heat source has a high and stable temperature
during heating season and it is fully available. Different heat sources have been analysed for the specific application in the school building. The heat sources being assessed in the study are ambient air, exhaust air, underground water and ground – soil.
Ambient air is the most common heat source for heat pumps. However, air – source
heat pumps attain operate in seasonal terms less favorable than heat pumps of other
sources. This is mainly due to fall in the capacity and the coefficient of performance
with decreasing outdoor temperature as well as the relatively high temperature lift and
an additional energy needed for defrosting operation. In general air-to-air and air-towater heat pumps seem to be not particularly suitable for complex heating, ventilating
and air conditioning (HVAC) installations. Thus, the evaluation has concentrated on
the ground source heat pump (GSHP). All types of ground source heat pumps have
been assessed, one with ground heat exchanger in the form of horizontal coils another
one with vertical U – tube arrangements and also with water wells. The horizontal
coils concept has been abandoned because of the lack of sufficient area on the site.
Neither the idea of water wells has proved to be a practical solution due to the necessity of having several water extraction and injection wells on the building site. Thus,
GSHP with the vertical ground heat exchanger is chosen as a heating or cooling source
for the building. During the heating season GSHP extracts the heat from the ground
through ground heat exchanger and supplies heat to building heating installations. It is
also possible to operate GSHP in mode of producing cooling effect. The fluid circulating in ground heat exchanger is used as heat sink for the condenser and the evaporator
cools the cooling water delivered to air handling units. The other heat source that can
be considered in this particular building is exhaust air in the ventilation system. The
ventilation type heat pump is basically heat recovery device in air handling unit. In the
study this heat pump type is called air-recovery-source-heat-pump, shortly ARSHP.
During the heating season ARSHP extracts heat from the exhaust air and heat the outside air. During the cooling season the heat pump operates in a reverse mode to cool
and possibly dehumidify outside air entering the air handling unit. The exhaust air
then cools the condenser coil at a lower temperature than the ambient air temperature,
ensuring better coefficient of performance. The AH unit is additionally equipped with
a recovery heat exchanger, thus the overall recovery process is more efficient and the
heat pump unit operates smoothly in most favorable temperature conditions. The favorable temperature conditions means that in winter time air temperature is higher on
the condenser side and lower on the evaporator side in summer time.
2. SYSTEM DESCRIPTION
The school building under the study is two – floor construction of 6100 m2 total
usable area. The building incorporates parts of different functions such as primary
school, kindergarten, canteen, community library and sports facility. The building has
been designed with a thermally efficient envelope. The high thermal effectiveness is
attained by applying very good insulation with the U-value of 0.18 W/m2K for walls
and roofs, and the U-value of 1.4 W/m2K for windows. Building areas as primary
school, kindergarten, canteen, community library and sports facility require different
indoor climate conditions. Thus, HVAC systems are chosen separately for specific
areas. Most rooms are equipped with radiator type central heating system with design
water temperature of 55 ºC, except kindergarten rooms where floor type heating system with design water temperature of 35 ºC is installed. All specific areas are served
by separate air handling units which in independent way supply air of determined
conditions. The heat losses of the building are covered entirely by central heating installations and AHU units are only supposed to treat outside air for attaining the proper IAQ parameters in ventilated rooms.
3. HEAT PUMP ARRANGEMENTS
The task of designing energy efficient system requires analysis of different configurations of heating and cooling plants serving heating ventilation and air conditioning
installations. The conventional solution for the considered building would consist of
gas-fired boilers producing hot water to cover heating needs and compression type
refrigeration units to cover cooling needs. This conventional option described as Case
0 can be replaced by two different configurations of heat pumps. Case 1,in fig.1, is an
application of the GSHP that delivers heat to central heating installations and heating
coils in air handling units (AHUs) during heating season. During cooling season the
GSHP operates in a reverse mode supplying cold water to AHUs with the ground heat
exchanger (GHE) serving as a heat sink for condensers. In this case one needs only
SHP units coupled with GHE to cover all heating and cooling demands of the building.
Fig. 1. Ground source heat pumps’ based HVAC
systems
Fig. 2. Ground source and air recovery source heat
pumps’ based HVAC systems
Case 2, fig.2, represents another configuration of heat pumps. GSHP supplies heat
just to central heating installations whereas air handling unit incorporating air recovery source heat pump (ARSHP) can heat or cool ventilation air. In this case both types
of heat pumps, GSHP and ARSHP, operate independently using fully their capacities
The systems’ model simulation has been carried out for both heat pump arrangements, Case 1 and Case 2. In Case 1 the COP of the GSHP unit varies in winter
and summer conditions. In heating mode COP takes the average value of 3 whereas in
cooling mode the average COP equals 5. The values of overall system COP are in
general 10 to 20 % lower than the unit COP because of the need of auxiliary power of
circulation pumps. Case 2 represents the situation where besides GSHP units operating
central heating installations there are also ARSHP heating or cooling ventilation air.
The GSHP units are smaller than in Case 1 but working with the same COP values.
The COP values for GSHP in Case 2 refer just to heating mode because the cooling
mode is realized by ARSHP. The COP values for ARSHP in AH units present quite
promising values, being on average 5,4 in heating mode and 4,2 in cooling mode. For
the practical reasons the COP values are time normalized for particular months. The
monthly average values are depicted in fig.3 and fig.4. Since the heat pump operation
is being assessed in the specific installation, the overall system COP is the better indicator than COP for just heat pump unit. The overall COP of the GSHP changes from
2.55 to 2,95 in heating season. In cooling season it varies between 4,8 to 5,2. The
overall COP for the ARSHP equals the unit COP and it changes between 5,0 and 5,4
in heating mode and it is in the range of 4,0 to 4,2 in cooling mode. The results of
COP calculations for design and part load conditions throughout the whole year are
used to assess seasonal performance factor (SPF). Heating and cooling loads of the
school building are calculated separately for central heating and air conditioning systems, thus, the GSHP and the ARSHP units are sized to cover precisely the particular
loads. Seasonal performance factors for cooling (SPFc) and heating (SPFh) are determined in separate way in Case 1 and Case 2. SPFh for the GSHP takes value of 2,60
and for the ARSHP it is 5,24. SPFc for the GSHP equals 4.1 and for ARSHP it is 4,2.
All the simulation results obtained for the system operation in Case 1 and Case 2 have
been used in comparison these two cases with the baseline system, Case 0, based on
an arrangement of the gas fired boiler and the compressor chiller with air cooled condensers. The comparative analysis give quite interesting results in terms of primary
energy usage and CO2 emissions in all
three cases. As for the primary energy
usage all three cases present the similar
picture with the smallest energy usage
in Case 2, as shown in fig. 3.
The calculated emissions of CO2 in
all three cases present a more complex
overview.
Fig. 3. Annual primary energy usage for three cases
One can evaluate CO2 emissions from gas – fired boilers in a straightforward way
knowing boilers efficiencies. An application of heat pumps carries additional difficulties concerning the type of electricity production.
The electricity production in a typical coal fired plants, that is prevailing situation
in Poland, causes considerable emissions of CO2. With the CO2 intensity
of 1,03 g/kWh the heat pump concept
is perhaps not so carbon saving technology, as is shown in fig. 4. Fig.5
presents annual CO2 emissions divided
by specific installations, central heating, ventilation heating and ventilation
cooling.
Fig. 4. Annual CO2 emissions for three cases
The electricity produced from gas- fired combined heat and power plants corresponds to much lower levels of CO2, the emission is smaller than in the case of gasfired boilers.
Furthermore, if heat pump
units are supplied by so called
green electrical energy, e.g.
wind produced energy, then the
CO2 emissions related to heat
pumps almost equal zero.
Fig. 5. Annual CO2 emissions divided by specific installations
4. CONCLUSIONS
• GSHP is able to satisfy all heating and cooling requirements of the building;
• GSHP – average COP (system) for heating mode is 2,55 and for cooling mode
4,25;
• ARSHP – average COP (system) for heating mode is 5,4 and for cooling mode is
4,20;
• SPF for GSHP in heating mode is 2,40 and 4,10 in cooling mode;
• SPF for ARSHP in heating mode is 5,24 and 4,20 in cooling mode;
• One can reduce capacity of GSHP and GHE by introducing ARSHP in AHU;
• Annual primary energy usage is higher in case 1 (GSHP) than in case 0 (baseline
– gas-fired boiler and chiller);
• Annual primary energy usage is lowest in case 2 (GSHP and ARSHP);
• Annual CO2 emissions are lowest in case 0 (baseline – gas fired boiler and
chiller), both Case 1 and Case 2 give higher emissions;
Heat pumps have worse environmental impact than conventional solution with
gas fired boiler because electricity is produced in coal fired plant;
Heat pumps could have much better environmental impact provided that the supplied electricity is produced in cleaner way, e.g. gas fired CHP plants or even better
wind energy;
REFERENCES
[1] BUGAJ A. Development of residential air-to-air heat pump for northern climate conditions. 2nd International Workshop on RESEARCH ACTIVITIES ON ADVANCED HEAT
PUMPS, Graz, September, 1988.
[2] BUGAJ A., MUSTAFE M. An ice storage concept in air conditioning system in office
building in Tripoli,Libya, XI International Conference Air Conditioning and Heating,
Wroclaw, June 2005.
[3] BUGAJ A., MUSBAH M. A heat pump concept in air conditioning system of an office
building in Tripoli,Libya, XI International Conference Air Conditioning and Heating,
Wroclaw, June 2005.
[4] BUGAJ A., MINIEWICZ M. Ecological and economic aspects of heat pump application in
a school building. Polish Journal of Environmental Studies, Vol. 17, No 3A 2008