The only sorption system for heating and cooling that is well established in the (Western European) market is the Robur pumped ammonia/water system. However this system is far bigger than the average house system. From sorption systems for houses only costs of prototypes, of future projections and of early market introduction (Nefit) are available. We based a cost curve on the Robur system (catalogue price of 11.810 euro at 36.6 kW condenser/absorber power = 330 euro/kW) and on information from the early market introduction and prototypes. In this way we came to a cost curve in which the price per kW is decreasing with increasing condenser/absorber power (see figure 8).

Table 1 gives the values of other cost parameters that were used for the calculation of the simple pay back time (pay back time without interest, depreciation and inflation).

In figure 9 we can see that with this cost structure a sorption system without solar system can only be cost effective for the existing houses (average and big) and not for the more energy efficient newly build houses (minimum and reference). In newly build houses the space heat demand is not high enough to make this system feasible. For the market segment of average and big existing houses a condenser/absorber power higher than 3 to 4 kW is not attractive.

When we add a solar system (figure 10) the simple pay back time becomes longer than 15 years for all options. In these calculations no tax credits or subsidies were accounted for. When there is already a solar system with the right dimensions available in the house, the pay back time of adding the sorption system becomes comparable to figure 9.

Again the system is more attractive for the average and big house (existing houses) and less for the minimum and reference (newly build). There is now a clear optimum for the average house (at 2 to 3 kW) and for the big house (at 3 to 4 kW).

Technology

There are numerous sorption processes that could be used for a solar sorption heating and cooling system. The most common sorption process that is used for cooling (LiBr/water) is however not suitable as a heat pump, because it cannot operate below 4 oC. The other common sorption process is ammonia/water. Recently in Austria the Solarfrost development was launched. This ammonia-water-hydrogen absorption/diffusion heat pump is especially designed for driving temperatures below 100 oC. The system can thus be driven by a standard solar hot water system. Other developments in ammonia/water in the Netherlands
are the Nefit diffusion/absorption heat pump and the Remeha pumped ammonia/water heat pump. These companies do not consider the adaptation of their heat pump for solar cooling. Another group of sorption processes that can be used is the sorption of vapour in solid materials like water/silica gel (Sortech) or water/zeolite (Vaillant)

In existing buildings the weight and the height of the system are important success factors (especially in the Netherlands, were we do not have cellars). In this respect the pumped ammonia system and the Sortech solid sorption system are potentially better than the Solarfrost diffusion/absorption system, because of the large pipe diameters at high pressure that are needed for the self pumping ammonia system. Moreover the diffusion/absorption system does not have the possibility to adjust from heating operation in winter to cooling in summer, because the condenser/absorber temperature is fixed by the hydrogen pressure.

A solar sorption cooling/gas driven heat pump is technically a feasible system (see also the work of IEA task 25 [Henning, 2004]). Because of the large number of circulation pumps that are needed (at least a solar pump, a generator pump, an evaporator pump and a condenser/absorber pump) parasitic power consumption is a point of attention in further developing the solar sorption system.

Safety

In The Netherlands there are no regulations for the application of ammonia in houses (at less than 2.5 kg of ammonia). However the application of ammonia at 15 to 25 bar can be hazardous when leakages occur in closed spaces (like attics or cellars). The water/silica gel of the Sortech system or the water/zeolite of the Vaillant system is in no way hazardous. The system pressure will stay below the ambient pressure (at condenser/absorber temperatures below 100 oC). Silica gel or zeolite is not hazardous.

Developments

Ecofys is one of the partners in the European Modestore project. In this project a technical test of the Sortech solid sorption system is performed in Germany, Finland, Austria and The Netherlands. The first system has just been installed in The Netherlands this spring and monitoring will be performed during the summer of 2004 and the winter 2004/2005.

Acknowledgement

This project was supported by the NEO (New Energy Research) programme that is implemented by NOVEM (Netherlands Agency for Energy and Environment) commissioned by the Dutch ministry of economic affairs.

Conclusions

• A sorption system without solar system can be cost effective for average and big existing houses. In newly build houses the space heat demand is not big enough. Sorption with solar for heating and cooling is not (yet) cost effective for the Dutch cost structure (without subsidy of tax credit).

• Optimal condenser/absorber power for cooling and heating is around 3 to 4 kW for the market segment of average and big existing houses. This is equivalent to an evaporator power of around 1 to 1.5 kW.

References

1. BAK, Survey natural gas use in houses in the Netherlands, Energiened, Arnhem, 2001.

2. Hennig H.-M. (ed.) Solar-Assisted Air-Conditioning in Buildings, Springer Verlag, Vienna, 2004.

3. Herold K. E. Absorption Chillers and Heat Pumps, CRC press, New York, 1996.