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small units, a Stirling engine. (See http://chp. defra.gov.uk/cms/gas-engines for an excellent overview of gas engines.)


Considerations for CHP applications The primary consideration should be whether the application requires the simultaneous need for electricity and heat over extended operating periods, typically between 5,000 and 6,000 hours a year. The longer the operating hours, the greater the economic and environmental benefit delivered to the end user. For larger CHP installations, typical applications would naturally include sizeable consumers of hot water and power such as hospitals, universities, large chemical plants and paper mills. However, social housing developments and smaller properties such as hotels, leisure centres, nursing homes, and sheltered accommodation can all provide suitable applications for mini-CHP.


Electricity to building circuits


distrib. panel


Exported electricity


3-phase grid supply


Elec.


exhaust pipe to exterior


heating return pipe


heating flow pipe


gas supply


Connect to 3 x 20 amp circuit breakers


Generated electricity


Engine start


Figure 4: Electrical integration of mini-CHP units


Electrical integration From an electricity generating perspective, the CHP unit should be selected to meet the site’s base load. This avoids or minimises the ‘spilling’ or exporting of electricity to the grid network, which at present on many sites would not offer a financially viable proposition. The key to maximising the economic benefit of the CHP scheme is to utilise all the electricity generated on site. When operating in parallel with the local


Figure 2: 5.5 kWe mini-CHP Unit installed in plant room of sheltered housing scheme


The mini-CHP unit would normally be selected to match the base heating load of the site (as shown in Figure 3) to maximise the running hours. The success of all forms of CHP depend on the integration with the building’s heating system – particularly with engine-based technologies. Matching the CHP unit to the base thermal load must be the priority (that is, the system should be sized on a heat-led basis and not primarily on the electrical load). The CHP unit must be installed as the lead heating appliance, with existing or new boiler plant providing additional capacity to satisfy peak demand.


Heat demand 100%


supply network, any electricity required above the output of the CHP unit would be imported from the grid network. For mini- CHP units that generate less than 16A per phase, connection and parallel operation with the electricity supply infrastructure should be carried out in accordance with Engineering Recommendation G.83/1 [1], which is more ‘fit-and-inform’, rather than a formal connection agreement with the local Distribution Network Operator (DNO). For those CHP products generating more than 16A, a formal connection agreement between the DNO and the end user is required under the requirements of Engineering Recommendation G.59/1 [2


]. Depending on the electrical output and the 50% Heating


Possible 2nd mini-CHP


1st mini-CHP Heating


Base load – e.g. hot water, swimming pool 1 2 3 4 5 6 7 8 9 10 11 12


Winter Summer Figure 3: Sizing CHP for base thermal load 52 CIBSE Journal August 2010 Winter


current produced, most mini-CHP units will generate 3-phase electricity. For example, a commercially available unit has an electrical output of 5.5kWe, at 415V, 3-phase, 50Hz. This equates to approximately 9A per phase, and as such is below the threshold of G.83/1. This unit generates electricity equally across the 3-phases. It is therefore important to ensure that the electrical load that is to be supported by the mini-CHP unit is also relatively well balanced across the three phases. Otherwise, in the event of an under-loaded phase (with the load say less than 9A in the case of this unit), electricity will be erratically ‘spilled’ on the grid with loss of benefit to the end user. This is true of both new-build and refurbishment markets, but potentially is more of an issue with the latter. If there is a 3-phase supply and distribution


Hydraulic integration The installation of a mini-CHP unit should be no more complex than a conventional heating boiler. The plant room should be ventilated, taking into consideration the combined outputs of the mini-CHP unit and all other heating appliances present. To maximise the annual operating hours, the CHP unit should be hydraulically integrated and controlled as the lead heating appliance. Care must be taken when considering the point of integration of the CHP unit flow and return connections.


If the water flow through the CHP unit is relatively low compared to the overall water volume in the system, then the CHP unit can be connected to pre-heat the return circuit water prior to entry into the main boiler plant. This results in the primary heating appliances not firing and their consuming less fuel to lift the water-flow temperature to the desired design set-point.


If the water flow through the CHP unit is relatively high compared to the system volume, then if connected as a pre-heat to the heating plant, there is a strong likelihood that the connection of the CHP will have an adverse effect on boiler efficiency, particularly if they are of the condensing type. Here reducing carbon emissions through the use of CHP would be negating carbon savings elsewhere in the heating system. In both Figure 5 and Figure 6 the CHP unit


has ‘access’ to both the heating and hot water loads via the low loss header. The ability of the CHP unit to serve both the heating and hot water loads is necessary to maximise the annual operating hours. Depending on the base thermal load of the building, it may be prudent to install a buffer tank between the CHP unit(s) and the heating/ hot water circuits. This is to prevent frequent cycling of the CHP unit during periods of low thermal load, which could lead to premature engine failure. For larger buildings, multiple mini-CHP


units can be installed, as in Figure 7, on the premise that there are sufficient thermal and electrical base loads to support the combined outputs. The installation of multiple units versus a single larger CHP product is supported by an increase in operational flexibility, provides redundancy and improves load matching.


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board in the plant room in which the CHP unit is installed, then electrical integration can be made locally rather than connecting at the main electrical incoming supply point, which may be some distance from the plant room.


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