Various approaches to solar cooling have been proposed to date, with single stage absorption cooling driven by flat plate or vacuum tube collectors dominating the market. The hypothesis here is that double stage absorption cooling fired by heat obtained from parabolic trough collectors has better economics for mid — and large scale applications. To conduct a high level verification of this hypothesis capital costs associated with the respective solar fields and chilling energy yields are compared for a system with a peak chilling capacity of 500kWR. In order to confirm the viability of the thermal approach in comparison with the direct photovoltaic approach an equivalent vapour compression system is also considered where the electrical power would be generated by a photovoltaic system.

Table 1. : Comparison of capital costs associated with the solar field required to achieve a chilling power of
500kWR for single stage, double stage and vapour compression with PV.

Capital costs for the solar field include delivery and installation. Costs associated with the preparation of the roof area to host the solar field structure are excluded. Capital costs for the chiller equipment and interconnecting pipe work and heat exchangers are not taken into account. In the same token on the PV side the inverter system and interconnection to the network are excluded. Efficiency figures are those applicable for generic products available in the market. For the single stage systems both a CPC and a vacuum tube collector system were considered, while for the double stage system the NEP Solar PolyTrough 1200 small aperture parabolic trough collector system was taken as a basis.

To estimate the annual cooling energy produced the three systems were modelled with varying degrees of detail for a location in southern France.

Modelling single stage system: Collector field energy yield is derived from general annual energy yield figures published in the industry and adjusted to site location

Modelling double stage system: The energy yield is calculated on the basis of an hourly model taking into account sun position, solar collector performance (optical material properties, thermal loss coefficients, IAM), chiller part load performance, inertia & heat-up losses as well as availability losses.

Modelling vapour compression system with PV: The annual cooling energy is approximated by the specific annual electrical energy yield for the photovoltaic panels times the average COP value of the vapour compression system of 3.25. The specific annual electrical energy yield for south of France is estimated at 1’450 kWhelec / kWinstalled.

The following table shows the results of the energy yields modelling for a location with the following site parameters (Marseille, south of France):

Latitude: 43°

Global radiation on a tilted plane: 1’865 kWh /m2 and annum DNI radiation: 1’775 kWh/m2 and annum

Table 2. : Comparison of the estimated cooling energy yield obtained from the 500kW single stage, double
stage and vapour compression systems for a typical meteorological year in the south of France.

From the first order capital cost comparison of Table 1 and the energy yield comparison of Table 2 it results that double stage absorption cooling is the most economical avenue for solar cooling. The ratio of cooling energy to solar field costs as proxy of economic attractiveness is almost double for the parabolic trough approach. The double benefits of higher COP of the chiller and good solar collection efficiency due to concentration and tracking result in an overall better performance of this combination.

It has to be noted that the fraction of direct radiation versus diffuse radiation in the total radiation available has an influence on the comparison between low temperature non-concentrating and high temperature concentrating approaches. The more the direct fraction of the radiation increases over the diffuse the more the use of concentrating collector systems makes sense. Maybe this partially explains why many of the solar cooling systems installed in Germany for instance use non concentrating collectors. With the solar cooling market developing in southern regions where DNI radiation is higher we expect to see a strong take-up of concentrating solar collector fields driving double stage chillers or similar high temperature / high COP refrigeration systems