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lation to electricity demand may need to be considered. The use of thermal storage and heat dumping may be of greater assistance in allowing the CHP plant a higher flexibility in matching demand profiles. There may also be opportunities for tri-generation (heat, chilling and electricity).

Some CH schemes are initially established without CHP, due to the large capital outlay involved. Even without CHP, there may still be environmental and economic advantages from load diversification, and further economic benefits from purchasing fuel in bulk. The use of pre-insulated pipes and low NO2, high-efficiency or condensing boilers have helped to make modern combined heat systems robust, environmentally friendly, and economically advantageous. It should be stressed, however, that CHP offers immediate environmental benefits, and long-term economic advantages.

3.5: Heat Distribution systems

After the building demands and the central plant location have been determined it is then possible to examine the heat distribution system. In order to analyse the network rapidly and to calculate costs, computer software is essential. For large schemes, only the primary mains, together with sample areas of sub-distribution mains, will need to be analysed at this stage. The more difficult aspect is to determine which routes for CH pipe work are technically feasible and cost-effective. 

Considerable assistance may be obtained by consulting ordnance Survey maps. These are now available on CAD format but at a significant cost. However, there is no substitute for an informed site survey, and a detailed examination of other utility services drawings. Within the feasibility study it will be necessary at least to identify any major constraints to routes, e.g. road and rail crossings, the presence of underground sewers, railway tunnels, etc. In selecting routes the use of open, grassed areas will lead to significant cost savings, as with the avoidance of major roads. Consultation with the Local Authority Planning and Highways Department is essential.

3.6: Optimisation

This is the most complex phase of a feasibility study, where the various options available for CHP plant, heat distribution and building heating systems need to be assembled into a number of scheme options — all with their individual sets of cost and revenue streams. Some cases may be simplified by a process of logical comparison, but normally it will be necessary to compare the options using discounted cash flow analysis, and hence to determine the preferred option on the basis of maximising net present value or internal rate of return. Some of the system optimisation issues that are likely to need evaluation are given below.

3.6.1: Operating temperatures

A high flow temperature will result in a larger temperature drop, lower flow rates and hence smaller pipe diameters can be used. This capital cost advantage will be offset by the need for more expensive building connections and possibly higher heat production costs, depending on the CHP plant selected.

3.6.2: Operating pressures

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