Description of installed system

1.1. System and operation concept

The system concept is shown in Fig. 1. The core components are the reversible adsorption machine which can be operated as a thermally driven chiller or heat pump and the borehole system that is used as heat rejection system for the chiller mode as well as the low temperature heat source for the heat pump mode.

In the summer operation mode (left schematic in Fig. 1) the adsorption machines works as a thermally driven chiller. It is driven by the heat from the solar collectors. A connection to the heating network of the building works as a heat backup in case of not sufficient solar driving heat. The system provides cooling to a cooling coil in the air handling unit, which is installed in the inlet air duct to the institutes’ canteen kitchen (3000m3/h). The waste heat from the chiller is rejected through the borehole system.

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In the winter operation mode (right schematic in Fig. 1) the adsorption machine is working as a thermally driven heat pump. It is driven by heat from the heating network of the institute, which comes form a CHP unit, and lifts low temperature heat from the boreholes to the useful temperature level. This useful heat is used in a heating coil to pre-heat the air in the main duct of the ventilation system (9000m3/h).

Fig. 1. System schematic with summer (left) and winter operation mode (right).

The system installed at the Fraunhofer Institute consists of a solar collector array of 20m2 with a 2m3 buffer storage and three boreholes of 80m each. An adsorption chiller ACS05 from the German company SorTech AG with a rated nominal cooling power of 5.5kW is used. This chiller is a pre-series development based on a previous prototype presented in [2].

For a general application in the residential sector this system layout offers the following advantages:

• The solar system is used through the whole year. In winter it is used for solar assisted heating and in summer it provides the driving heat for the cooling system.

• The reversible adsorption machine is also used during the whole year: as a heat pump in winter and as a thermally driven chiller in the summer period.

• In the heat pumping mode the adsorption system enhances the energy output of the driving heat source through the use of ambient heat from the boreholes.

• The borehole system also serves two purposes: first as a low temperature heat source in winter and second as a heat rejection system for the chiller in summer. Thus the boreholes are not only used year round but are also regenerated in the summer season.

• In favourable conditions and periods of low cooling power requirements, the installed boreholes can be used for direct cooling.

Nevertheless, neither the direct solar heating nor the direct cooling with the boreholes has been implemented in the present system.

2.2. Control procedures

The operational concept foresees an operation only during weekdays. The operation conditions are as follows:

Cooling operation is carried out when:

• the inlet air temperature exceeds 20°C (2K hysteresis),

• the air temperature in the kitchen is above 23°C (2K hysteresis),

• the time is between 6:45 and 16:00 o’clock.

Heating operation is carried out when:

• the inlet temperature in the main air duct is below 14.5°C (3K hysteresis),

• the inlet air temperature is above 3°C (freeze protection of the machine),

• the time is between 6:45 and 19:00 o’clock.

Solar heat is used whenever the mean temperature in the upper part of the storage is above 73°C with a 5K hysteresis for turning of solar heat supply.

In spring and autumn it may happen that the air temperature falls below 14.5°C in the mornings and thus the heating mode is activated, but later during the day temperatures in the canteen kitchen rise above the threshold for cooling operation. In these cases the system is operated in the heating mode first and later in the cooling mode. This operation is called ‘alternate mode’.

The volume flows in the three circuits are kept constant and correspond to the nominal flows required by the chiller. Energy efficient pumps have been installed and the flow rate is set via the three power steps of the pumps.

3. Data acquisition and evaluation

A new prototype of the reversible adsorption machine was installed in spring of 2007 and is in operation since then. In the summer season of 2007 about 282 hours of operation in the cooling mode have been monitored. Evaluation is carried out at three levels:

1. performance of the chiller as a component

2. evaluation of the operation conditions of the whole system for system optimisation

3. evaluation of the overall systems energetic performance

These three evaluation levels address each a different group: while the first is mainly important for the chiller manufacturer in order to decide if the machine is working as expected, the second level targets the system developers which are interested in the optimisation and smooth operation of all system components in order to optimise the energy efficiency and thus produce the highest possible savings for the end-user. This result is covered with the third evaluation level.