Planning Phase

The first and often most difficult stage of planning a solar cooling plant is to calculate the actual cooling demand. Norm calculations usually give the sum of multiple single peak loads resulting in a very high overall peak load. This demand however will only be required for a very small number of running hours. This explains why electrical cooling machines are mostly over-sized and therefore have to continuously modulate. This unstable operation leads to lower efficiency levels and unnecessarily high power connections.

If a solar thermal cooling plant was to be designed based on these peak loads, the plant would not only be very expensive, the over-sizing also leads to operational problems.

Table 1: Overview of the thermal cooling plants already operational from S. O.L. I.D.

Location/Project

Cooling Machine

Constr.

Cooling

Power

Collector

Area

Pristina — EAR Tower

2 Pcs. LiBr-Chiller

2002/3

70 kW

226 m2

Leutschach — Wine Cooling

Ammonia, Custom — made (Podesser)

2003

15 kW

100 m2

Graz — Buro, Test Plant

Ammonia, Custom — made (Kunze)

2003

2 kW

8 m2

Stadtwerke, Crailsheim

1 Pcs.. LiBr-Chiller

2004

15 kW

Energy from boiler

Brussel — Renewable Energy House

1 Pcs.. LiBr-Chiller

2005/7

35 kW

60 m2

Phonix — Desert Outdoor Center

1 Pcs.. LiBr-Chiller

2006

70 kW

168 m2

Qingdao — Olympic Village

2 Pcs.. LiBr-Chiller

2006

512 kW

631 m2

Tampa — Estellas Restaurant

1 Pcs. LiBr-Chiller

2007

70 kW

210 m2

Lisbon — CGD

1 Pcs. LiBr-Chiller

2008

585 kW

1592 m2

Phoenix-Lanta

1 Pcs. LiBr-Chiller

2008

130 kW

504 m2

Gleisdorf- Service Center Municipality

1 Pcs. LiBr Chiller, and DEC

2008

35 kW 6000 m3/h

260 m2

Graz, office

1 Pcs. Li Br Chiller

2008

17.5 kW

58 m2

All of the cooling projects for air-conditioning purposes were over-dimensioned by a factor of 2-3 during the initial design phase. According to the DIN, the EAR Tower in Pristina had a cooling demand of 210 KW. The actual cooling system installed by S. O.L. I.D. uses two cooling machines with a total nominal capacity of 70 KW (peak 90 KW) and a back-up 30 KW compression cooler. The maximum measured cooling demand over six summers (including the particularly hot summer of 2003) was 80 KW. Same has been monitored on all other projects.

The influence factors on the cooling demand are as well as people, IT equipment, lighting and ventilation, most notably passive solar gains.

A realistic estimate of the cooling demand requires further experience or a simulation programme.

The cooling demand also varies considerably depending on ventilation, in particular rates of air change and heat recovery installations play a big role.

For air conditioning in office buildings, a ratio of 12m2 collector area per 100m2 of office area to be cooled has proven to be relatively accurate. Larger collector areas are necessary only in extreme climates with high cooling demands at night.

Diagram 1: shows measurements from a typical conventional cooling machine in the USA. Notable is that during over 90% of the running hours, the cooling demand was lower than 20% of the installed capacity,

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and 99% of the time lower than 30%.

Diagram 2: This building has a nominal load of 1200 tons, however, even in peak seaseon operation is

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mostly between 250 and 500 tons.

Diagram 3 Cooling demand in different Austrian buildings (Source: Fink et al, AEE Intec)