None of the projects would be satisfying operational without detailed support and optimisation in the beginning over a period of 3 to 6 month. The supervision and adjustment of set points and flow rates has significant impact on the performance and is even for experienced people difficult to foresee.

All of the plants have proven to operate reliably. The Cooling machines achieve the COP detailed in the manufacturers’ specifications during constant operation. Low return temperatures from the cooling tower often allowed a lower flow temperature from the solar plant to be used to drive the cooling machine than that specified by the manufacturers.

The offset between maximum solar radiation and the maximum cooling requirement is approx. 2-3 hrs and can be easily covered using the buffer tank.

The efficiency of conventional flat plate collectors, as used for warm water production is noticeably reduced in this application. Flow temperatures up to 100°C on the primary side of the heat exchanger, result in a lowering of the collector efficiency curve. In large plants it is also important to balance the collector hydraulics.

In the last projects, we used a dramatically improved large area flat plate collector, which in the required temperature range collects approx. 20-30% more solar energy. The economy of the entire concept is thereby much improved. This collector has just passed certification and actually is ahead of not only flat panels, but as well significantly better performing than vacuum tubes at temperatures of 100° C. Practical experiences show we can reach solar gains of app. 750 kWh/m2 in the Arizona project on a mean temperature level for collector operation of 80 to 90°C.

The modifications to the collector include more measures towards reducing the heat loss: a Teflon sheet above the absorber plate reduces top losses, increased insulation in the rear panel and an especially airtight frame also improve performance. These new collectors also offer potential for application in district heating and process heat.

The cooling load can often be entirely covered by the solar plant when we have availability of space for solar panels — particularly on sunny days. The back up is only required on days with little solar radiation. However, the peak electricity demands on the mains supply and distribution network due to cooling requirements don’t generally occur on these days.

Absorptions chillers have proven to be really stable. Not only mishandling though local personal never affected them seriously, I actually found chillers in operation since more than 70 years that are still operating sufficiently.

Diagramm 7 shows average solar gain per day used for cooling (blue column) and
heating (red) in DOC project in Arizona.