Heat Output
Heat is available from a gas engine as:
- Exhaust gas at 400°C (up to 600°C from a gas turbine).
- Engine cooling water at 80°C.
Connection to the site heat system should be at its lowest temperature possible (normally the return feed to the boilers) to maximise the heat recovered. Swimming pool heating is an ideal use for the heat generated by a CHP unit. Alternatively, a connection may be made to the boiler return of a low temperature hot water (LTHW) system.
Where steam is the site heat transfer medium, the CHP may generate steam directly in the exhaust gas heat exchanger. In this case, only a proportion of the engine heat will be recovered.
In all cases where heat is recovered from the engine cooling circuit, a heat exchanger will be fitted between the engine and site circuits, keeping the two water circuits separate.
It is important to consider heat distribution in conjunction with heat recovery, as the distance between the heat load and the CHP package influences the cost of the pipework needed to connect the CHP unit to the site.
Unless the exhaust gases are used directly for heating or drying, the heat output from packaged CHP units is usually in the form of hot water. The heat is transferred to the user via a closed loop ‘flow and return’ pipework system. The ‘flow’ pipework delivers hot water at about 80°C to the point at which heat is transferred to the user. The water then passes into the ‘return’ pipework and is returned to the CHP package at a temperature that is about 10°C lower than the flow temperature. The closed nature of the loop means that the hot water is not used directly but acts as a heat transfer medium. This allows the addition of small quantities of chemical to the water to improve the system’s resistance to frost and corrosion.
In many cases, the CHP package will be installed on a site where a hot water flow and return system already exists, and it may be possible to make connections to the appropriate parts of the existing circuit. There are essentially two ways of connecting a CHP unit in this situation:
- In series, as a bypass in a suitable return to the boilers.
- In parallel with the boilers with conventional boiler plant.
Connection in series is most frequently used with existing installations, since it creates the minimum interference with existing flow and control arrangements. Connection in parallel is preferred for completely new installations, especially where the CHP unit is likely to supply a significant proportion of the total heat load. In both cases, it is usually possible to connect the CHP package into the existing heat system in such a way that it adds its heat upstream of the existing boilers or water heaters. The existing boilers then operate as top-up or stand-by facilities for the CHP plant.
When the heat output from a CHP package cannot be used on the site, and power output must be maintained, a cooling system needs to be incorporated within the flow and return pipework. This is often referred to as a ‘dump radiator’ and is normally controlled by a valve connected to a temperature sensor on the return water inlet to the CHP package. If the water temperature exceeds a set level, the valve opens to pass water into the dump radiator and directly back to the return pipework system.
In order to comply with the requirements for Good Quality CHP and calculate the Quality Index, a suitable method of measuring the heat energy supplied to the site (rather than dumped) must be provided.
The hot water system must be designed to achieve the rates of flow and the return water temperature that will allow continuous operation of the CHP package. The system pipework must be of the correct diameter, and it must incorporate sufficient pumping capacity to maintain the correct flow and temperature conditions. It is common to equip the system with duty and stand-by pumps to ensure maximum availability. Furthermore, the pumps must be selected to operate with the dump radiator system in full or partial use, or with the hot water flow all passing to the site. It is also important to ensure that heat distribution systems have sufficient levels of thermal insulation to prevent heat loss and minimise hazards. The system must also incorporate the means of isolating individual plant items for maintenance, while allowing others to continue operating.
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