DHC District Heating and Cooling

DISTRICT AND COMMUNITY HEATING

DHC is an integrative technology that can make significant contributions to reducing emissions of carbon dioxide and air pollution and to increasing energy security.

The fundamental idea of DHC is simple but powerful: connect multiple thermal energy users through a piping network to environmentally optimum energy sources, such as CHP, industrial waste heat and renewable energy sources such as biomass, geothermal and natural sources of heating and cooling.

District heating is traditionally divided into two types namely that serving flats, apartments and housing, and that serving a variety of building owners that can include commercial, retail, industrial, residential and local authority. The former is normally called community heating and the latter district heating.

Both district and community heating in recent years have been associated with power generation in the form of combined heat and power plant (CHP) where the electrical power generated is used locally and any excess power supplied to the national grid.

In one community heating system using CHP, blocks of flats are heated from a central plant and the lifts, entrances, corridors and landings are powered and lit from the electricity generated. Waste incineration is used to generate heat and power in district heating as also is biomass.

The focus for district and community heating schemes is clearly heating, so, the heat load for the scheme will be the major component.

The effect on overall efficiency, when the CHP option is used, is to raise it from 30% to as high as 75%. It is necessary however for the generator to find a customer for the heat generated from the CHP plant. Community and district heating can also benefit from the application of CHP technology as electricity can be used to power the plant with the surplus sold to the national grid.

The two main investments in a district heating system are the heat production plant and the network. The heat production plant is a single investment; the cost of it depends on the total annual amount of heat load of the area. The investment in the pipe system, otherwise, is a question of the length of the pipe network within the area of the heat supply and therefore is dependent on two dimensions: thermal length and thermal width. Therefore it follows that the costs of the distribution network can vary appreciably for different network geometries and type of systems.

Factors for consideration:

The feasibility study for a new scheme will need to investigate and research a number of areas before and initial decision can be reached.

- Type and concentration of consumers: the greater the variety the better, and could include commercial, industrial, retail, residential, schools, libraries, museums, leisure centres...

- Base load consumer: this would provide a fairly constant and substantial requirement for heating throughout the year ensuring continuous use of plant. Another base load usually available is the need for hot water supply particularly in residential sector, hotels, hospitals, campings and sport centres.

- Consumer acceptance: a major factor is the charge for the service which needs to be at least 10% below the cost for running and maintaining individual boiler plant. Another factor would be the security of heat supply.

- Environmental factors: ideally the heat and power generating plant needs to be located near. However, this mitigates against the noise disturbance and pollution which may result.

- Energy utilization: a district heating proposal shows a clear advantage in its use of energy from fossil fuel, waste and renewable sources over local heat generation.

- Maintenance philosophy: one of the lessons learnt in early district heating schemes is the importance of getting the maintenance of plant and external pipework right. A sure way to achieve this is to engage the contractors responsible for the installation of the distribution mains, for example, in a maintenance contract, following completion.

CHOICE OF SYSTEM AND OPERATING PARAMETERS

Energy sources available: These would include the most likely energy sources for the scheme - fossil fuel available, type of waste and type of renewable energy.

Heat supply ratios: Assessment of approximate ratios of process, heating, hot water supply loads and power loads are needed for the feasibility study. This will have a direct bearing on the choice of plant.

Heating medium: The heating medium needs consideration. It is invariably water because of its high specific heat capacity and may be LTHW, MTHW or HTHW. Steam might be considered at the central plant because of its high latent heat.

Heat exchange: If high temperatures are employed from the central plant it will be necessary to reduce the temperature using heat exchangers at substations. Industrial consumers may want high temperature distribution for their process and space heating.

Consumer supply: The distribution pipework layout should seek to ensure a ring main arrangement, so that, if a consumer's supply fails, a temporary reconnection can be made quickly.

Basis for charging: There are three ways in which the consumer can be charged for energy use:
    - Flat rate, which avoids the use of metering equipment and hence its initial cost and maintenance (not recommended).
    - Service charge plus charge for energy actually used.
    - Charge for energy use that includes the service charge.
It is important for avoiding consumer concerns and providing a good and reliable service that the instrumentation installed must be carefully selected both in the accuracy of the metering equipment as in the recording process.

Future requirements: It needs to take in consideration future extensions to the scheme and make a strategic plan.

Diversity: The feasibility study will address the diversity factor that will be applied to the plant for sizing purposes. Clearly the central plant is not sized on the total net load when all consumers are on line. As a general rule, the lower the number and diversity of consumers connected to the scheme the higher will be the diversity factor. The CIBSE Guide book A recommends a diversity factor of 0.7 for district heating schemes that have a wide spread and high number of consumers.