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The following issues should be considered at an early stage of design as part of an evaluation of the viability of CHP.

Organising a connection to the grid

Unless a CHP engine can be connected to the grid, its size may be significantly reduced. This is so that demand for electricity across a development is not a constraint on the engines daily running.

A licence to connect to the grid must be provided by the distribution network owner (DNO). A G83 covers the smallest units under 6kW, a G59 licence is for medium units up to 5MW and a G75 is for up to 50MWh. The operator will not necessarily be under an obligation to provide this licence.

Sale of electricity back to the grid will be to a different entity, the electricity supplier. As the sole provider they are in a strong position to negotiate low rates for the sale of CHP output back to the grid. Any estimate of payback periods should account for this.

In some instances the local grid infrastructure may not be sufficient to handle the feed-in of significant power. The  developer may be asked for a financial contribution to upgrade infrastructure.

A scheme that has a large CHP engine conditioned as part of its application, but cannot then connect to the grid may find itself with a near un-resolvable problem.

An associated issue arises where there is no grid connection and where the development has several branches of electrical supply. It is likely that CHP can only be synchronised with a single supply.

Early stage discussions with both the DNO and Electricity Supplier are recommended.

Rating of CHP Engines

CHP specification in pursuit of CO2 reductions often means that systems are rated to maximise their contribution. Because electricity may be fed to the grid, but there is a limit to the demand for heat, thermal loads often act as the principal constraint. CHP engines do not modulate well in terms of thermal output and this means that a CHP engine must be rated with respect to peak loads and understanding of the way in which daily demands vary throughout the year.

It is often the case that early in the design there is insufficient detail about daily heat demands to arrive at a final rating for the CHP engine.

A starting point is to consider the year round base heat load of the development, often equivalent to the demand for hot water. For a residential scheme a simple rule of thumb is 0.5kw per dwelling. Thermal storage will be required to hold back heat output by the CHP engines and smooth out peaks in daily demand. For good stratification, a thermal store should be 1.5 times as tall as it is wide. For volumes of water above 8 cubic meters, this means that storage may exceed the 2.8m floor to ceiling height that would typically be available in single storey plant room.

Cost Considerations

The CHP engine itself ought not to represent a significant proportion of capital outlay. A ‘micro’ 25kWtherm CHP engine should be available for around £45,000.

Other physical components that may be required are heat and electricity meters, protective enclosures, electrical protection and earthing equipment, power modulation boards,

Higher costs for CHP are associated with the intangibles – the export licence, poor feed in tariff rates which lengthen the payback period, maintenance contracts and the opportunity cost of the loss of saleable floor area. If a basement has to be excavated to accommodate a plant room, then costs will rise into the millions.

Plant and Services Implications

Heat recovery in the CHP does not capture all low grade heat that is produced. Where CHP is sized to provide an electrical load rather than a thermal load, it may be that some degree of heat dumping is necessary.

A mechanism for heat rejection must be incorporated into the design of systems. On smaller packaged units this may be part of the unit itself. For larger systems above 300kWtherm separate heat rejection may be required.

Ventilation of the CHP unit will require increased air flows to plant rooms. Where there is restricted external wall space for louvers (eg in a basement plant room or for plant at the centre of a building) then a forced ventilation strategy may be required.

The motor at work in the CHP engine means that there is continuous mechanical friction. This means the life time of the CHP engine is shorter than for a more passive system such as a boiler, and they are more prone to breakdown. Regular servicing after a 1,000 hours operation would be part of a typical maintenance schedule, and system can be remotely monitored to keep track of performance.

Fuel Considerations

Gas is overwhelmingly the fuel of choice for CHP engines. However, there are alternatives that can offer increased CO2 saving.

Biofuels are low carbon, but not without complications. Biofuel from plants for instance may be created at the expense of displacing crops for food production.

These are liquid fuels requiring different ignition to the gas system, so retrofitting is not possible. Variable quality of the fuels can mean that the lifetime of engines is reduced and more regular servicing is required.

Liquid fuels will be the right choice in some circumstances but are not a common strategy.

Footprint of CHP

Though the footprint of a CHP engine increases with it’s rating, engines will fall into size bands that are associated with the particular diesel engine at the heart of the system.

Therefore a higher CHP rating need not necessarily mean a larger footprint, unless that rating falls over a certain threshold. Furthermore footprint per kWH output is much lower for higher rated systems.