How to make economics stack up

As individual systems this may in some cases be true; however if the services are combined, a cooling system installation can achieve a COP of 16 or better, and the heating efficiencies can be improved from 91% to 200% or better.

A design of mine nearing completion in the North-west has combined the ground source cooling facility with a rainwater harvesting system and achieved the efficiencies above. When looking at the capital cost for providing boreholes or closed loop circuits for cooling facilities, both were uneconomical. By looking at the cost of providing rainwater harvesting tanks larger than required for just flushing toilets, a relatively economical cost was produced. Storage was calculated to provide six hours thermal mass of water for the peak load on the building, then with the tanks buried at a depth of 5m, overnight the water stored would be 10ºC (ground temperature).

The diagram above indicates how the COP assessment was made. The actual design and calculations were a little more complex, but the underlying principles are that the client gets a system that is cost-effective both in running and capital expenditure. The actual COP/

efficiencies cannot be fully measured until the systems are up and running.

A future chiller has been incorporated for extreme weather conditions where no or insufficient rainfall can be collected to operate the ground cooling facility as designed. This will provide comfort to the client who may be concerned regarding the predicted very hot, dry summers ahead. Design consideration has been given to local rainfall data and how much water will be needed for sanitary requirements.

William Gladstone ACIBSE MSoPHE