Air cycles in buildings 

 

 
In 1995 BRE and FRPERC carried out a small scoping study to explore the opportunities to exploit the environmental benefits of air cycle technology in buildings. The work concluded that the viability of air cycle cooling systems in buildings, in terms of energy consumption and associated power station CO2 emissions (the system is powered by an electric motor), depended on being able to make use of cooling and heat recovery simultaneously for as much of the operating time as possible.  This tends to favour building types such as hospitals and hotels rather than commercial offices. 

 

Proposed building air cycle

A small follow up study has provided a more detailed design and performance specification for a hypothetical building air cycle plant with the following capability: 
  • to provide chilled water at 6oC; 
  • to provide heat recovery (either concurrently with cooling or separately) to heat water to 81oC; 
  • to operate under all UK ambient air temperatures (approximately -3oC to 28oC); 
  • to operate between 10% and 100% of full load, heating or cooling; 
  • to be a 'packaged' unit with simple electrical power and water pipe connections; 
  • to be sized to provide a nominal 100kW cooling in summer. 
 
 
 Click on icon to down load proposed building air cycle system schematic
 
  
A proposed air cycle system to meet the above specifications is shown above. It comprises a closed circuit around which air is driven by a primary centrifugal compressor driven by a switched reluctance motor, and a secondary 'bootstrap' centrifugal compressor and expansion turbine mounted on the same shaft. Electronic speed control is an inherent feature of switched reluctance motors. The basic cycle configuration follows that used by conventional aircraft cabin air conditioning systems. 

The system has two load air to water heat exchangers, to provide heating of hot water to 81oC and cooling of chilled water to 6oC.  The heat of compression is taken out of the circuit by a heat exchanger downstream of the bootstrap compressor to heat water for the building space heating or hot water services.  Cold air is discharged from the expansion turbine at about -15oC and is delivered to a heat exchanger for cooling chilled water. 

A vital component is the recuperator which is placed between the hot and cold sides of the circuit and effectively increases the working temperature difference. This enables sub-zero temperatures to be achieved from the expansion turbine outlet to cool chilled water to 6oC, and temperatures over 100oC at the compressor outlet to heat water to 80oC. 

Progress on building air cycle system

 The main rotary machinery components were delivered to FRPERC in April 1999, and the design for the building air cycle system has reached an advanced stage.  The next stage is to finalise the heat exchanger specification and procure them.
 
Andrew Gigiel, Steve Russell, Keith Dunsden and Phil Jones (from left), inspect the newly arrived powered compressor and bootstrap compressor/expander units. 
 

The rotary machinery components were donated to the project by Allied Signal Noramalair Garrett Ltd.

 

Performance

Computer modelling of the proposed system shows that ambient temperature has an insignificant effect on the system performance when simultaneously providing heating and cooling, at near full load and part load, see Table 1. In comparison the cooling performance of vapour compression systems reduces at high ambient temperatures and part load. Table 1 also shows the electric power input (including the ambient air fan when heat or cooling is being rejected), and corresponding COPs (coefficient of performance) for heating and cooling. These are relatively low compared with those achieved by vapour compression systems operating at 'conventional' temperatures. However, it should be realised that vapour compression systems are normally incapable of providing the elevated temperatures necessary to provide useful heat recovery, and the efficiency of vapour compression cooling systems is also markedly reduced when operating at high ambient temperatures. 

 

  

Table 1  Predicted building air cycle system performance

 
+28oC ambient
+15oC ambient 
-3oC ambient 
 
80kW 
cooling 
20kW
cooling 
80kW 
cooling
20kW 
cooling 
80kW 
cooling 
20kW 
cooling
Heating output kW
205.8 
 66.3
207.9 
 66.3 
205.8 
66.3 
Shaft input power kW
128.7 
47.0 
128.1 
47.0 
128.1 
47.0 
Electrical input power kW
139.9 
56.0 
139.2 
56.0 
139.9 
56.0 
COPcooling
0.57
0.36
0.57 
0.36 
0.57 
0.36 
COPheating
1.47 
1.18 
1.49 
1.18
1.47 
1.18
 
 
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Prototype aircycle refrigerator

FRPERC fitted an open air cycle system to this domestic freezer to demonstrate the feasibility of using air as refrigerant, instead of environmentally harmful CFCs.  The building air cycle system will be constructed by modifying a train air-cycle air-conditioning unit donated to the project by Allied Signal Normalair Garrett.
 
 
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