Chapter 5: Thermally Driven Heat Pumps for Heating and Cooling
_______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
197
OPERATIONAL EXPERIENCES OF A TDHP SYSTEM FOR SOLAR
COOLING AND HEATING OF A CANTEEN
Matthias Schicktanz, Florian Mehling, Fraunhofer-Institute for Solar Energy Systems ISE,
Department Thermal Systems and Buildings, Heidenhofstr. 2, D-79110 Freiburg, Germany,
Florian.mehling@ise.fraunhofer.de
Abstract: Solar cooling systems offer the possibility to use primarily the renewable energy
source solar heat for space cooling. Moreover, a solar cooling system can also be used to
provide heat for space heating, in the case of working in heat pump mode. Besides
theoretical investigations practical experience is required to evaluate the performance of
solar cooling in the field. This document presents the main results from a solar cooling
installation for canteen cooling, which is in operation since 2007. The data evaluation shows
a seasonal electrical performance factor of 26.1 for space heating and 6.5 for space cooling.
A seasonal thermal performance factor for cooling of 0.4 has been determined.
Key Words: adsorption chiller, solar cooling, operational experience, seasonal
performance figures
1 INTRODUCTION
This report is about a thermally driven heat pump (TDHP) system operating in cooling as well
as in heating mode. It is installed at the canteen of Fraunhofer ISE in Freiburg, Germany.
This system is in operation since 2007. The main purpose is to provide air-conditioning for
the kitchen of the canteen. If capacity is available the system supports the building main
system with heat for the lobby. Depending on the inlet air temperature the operation between
cooling and heating can switch during a day. For example, heat for space heating can be
provided in the morning and afternoon while at noon the system switches to chill the kitchen.
The solar collector field of 22 m² aperture area is installed at the roof of the building and a hot
water buffer storage of 2 m³ is installed in the basement. From the hot water buffer storage
heat is taken to power the thermally driven heat pump system. Heat from the heating network
of the ISE institute can be used as backup. Three bore holes are a part of the system. In
cooling operation these holes reject waste heat to the ground while in heating operation heat
is taken from the ground.
2 GENERAL DESCRIPTION OF THE SYSTEM
The ACS08 from the manufacturer Sortech AG is an adsorption type thermally driven heat
pump. The rated chilling capacity in cooling operation at 15°C is about 8 kW, while the
heating capacity in heating operation at 30°C is approx. 20 kW.
The 2 m² hot water storage with a stratified charger is only heated by the flat panel collector
field. An external heat exchanger between the storage and collector field is used to separate
the brine from the storage.
While the system is operating in cooling mode the rejected heat is pumped to three terrestrial
probes, which are acting as a heat sink. Each of those heat probes is installed vertically into
Chapter 5: Thermally Driven Heat Pumps for Heating and Cooling
_______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
200
operator and thus allows a flexible data management. The post processing of the raw data
further reduces the values to hourly accumulated and mean values – depending on the
quantity considered. For the hourly mean temperature values also a standard deviation is
calculated in order to judge the stability of the temperature within the evaluated hour.
4.2 Cooling Operation
In cooling operation the system shows a seasonal thermal performance factor of 0.40. Thus,
per unit of consumed heat 0.40 units of cold were produced. However, this seasonal value
includes all disadvantageous operation states. As shown in Núñez (2008) the seasonal
thermal performance factor has a value of 0.57 when neglecting the disadvantageous states.
With a more sophisticated operation strategy the seasonal performance factor can be
increased.
The seasonal electrical performance factor of the system is 6.5. This means that per unit of
electricity consumed 6.5 units of cold were produced. The system still has potential to
increase this parameter.
The following reasons are responsible for disadvantageous operation states:
The temperature of the institutes heating network drops down from time to time.
These fluctuations are not caused by TDHP system. The lower or fluctuating driving
temperatures lead to a reduced seasonal performance factor of the TDHP.
The performance calculation takes into account all starts and stops. Many such
operation times occur which favour disadvantageous operation states. The short
operation periods could partly be avoided by implementing a controller with
forecasting strategy and the usage of chilled water buffer storage. Partly, the short
operation times cannot be avoided if they belong to user behaviour.
The design of the hydronic system was sized for a smaller cooling capacity chiller in
the first place. However, the TDHP was replaced by a later system with higher
capacity. While the installed hydraulic system couldn’t been modified for the higher
nominal flow rates of these later generations, the pumping capacity had to be
increased to meet the higher flow rates. This fact led to a higher electricity
consumption and thus to a lower seasonal electrical performance factor of the
system.
4.3 Heating Operation
The seasonal thermal performance factor for heating operation is 1.24 and the seasonal
electrical performance factor is 26.1. As shown in Núñez (2008) the seasonal thermal
performance factor of the system in heating mode is 1.43 when neglecting disadvantageous
states. The reduction of efficiency from 1.43 to 1.24 is caused by the same reasons as
described above.
5 SUMMARY
The seasonal performance figures of a solar cooling system were presented. In cooling
operation the seasonal electrical performance factor is 6.5 and the seasonal thermal energy
efficiency ratio is 0.4, though can be increased to 0.57. In heating operation the seasonal
electrical performance factor is 26.1 and the seasonal coefficient of performance is 1.24. It
was already proven that the last factor can be increased when avoiding disadvantageous
states.
The results show that a reliable operation of the solar cooling system is ensured. However,
the heating network of the ISE institute used as backup shows irregular behaviours which
Chapter 5: Thermally Driven Heat Pumps for Heating and Cooling
_______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
201
affects the performance of the system. This raises the question how necessary operation
conditions can be assured in applications with different needs to the supply system.
Moreover, frequent start/stops may be avoided with a better control strategy and will result in
a better performance. It becomes clear, that a high potential lies in controller development.
Future research should address this issue.
6 REFERENCE
Núñez, T., B. Nienborg, Y. Tiedtke 2008. “Heating and Cooling with a Small Scale Solar
Driven Adsorption Chiller Combined with a Borehole System”, Proc. 1st Int. Congress on
Heating, Cooling and Buildings (EUROSUN 2008), 7th to 10th October, Lisbon, Portugal.
