Combined Heat and Power (CHP) is the simultaneous production of heat and power from a single fuel or energy source, at or close to the point of use. An optimal CHP system is designed to meet the heat demand of the energy user – whether at building, industry or city-wide levels – since it costs less to transport surplus electricity than surplus heat from a CHP plant. By using the heat output from the electricity production for heating or industrial applications, CHP plants generally convert 75-80% of the fuel source into useful energy. For this reason, CHP can be viewed primarily as a source of heat, with electricity as a by-product; hence the location of the CHP Waste-to-Energy plant is quite crucial since it has to be close enough to its heat consumers.
CHP has a long history within the industrial sector, which has large concurrent heat and power demands, and in the district heating sector in countries with long heating seasons. It can cover heat demands in residential, public and commercial buildings, as well as process steam demands for industrial users.
Combined Heat and Power Generation
When producing a combination of heat and power, it is possible to use up to 80 percent of the energy of the waste. With a boiler designed for waste incineration (moderate steam parameters), an output of electricity at 20 to 25 percent and an output of heat at 60 percent can be achieved. When a combination of power and district heating is produced, a so-called back pressure turbine is used. The back pressure is determined by the temperature and the flow of the coolant, which is usually water from a district heating network.
A heat exchanger serves as interface between the district heating network and the building’s own radiator and hot tap water system, so as for the hot water used in the district heating system not to be mixed with the water of the customer’s network. District heating piping networks consist of feed and return lines, which transfer hot water via well-insulated pipes to the customer premises.
In countries with lower heat demands but higher cooling demands annually, a supplementary district cooling solution should be preferred. District cooling is the centralized production and distribution of cooling energy, which is a sustainable alternative to conventional electricity or gas-driven air conditioning systems. Chilled water is delivered via an underground insulated pipeline to office, industrial and residential buildings to lower the temperature of air passing through the building's air conditioning system. Absorption chillers use district heating as an energy source to cool a district heating circuit. Large units can be placed centrally to supply large district cooling systems, while small units can be located in buildings that require cooling and connected to a local cooling system.
Combined Process Steam and Power Generation
When producing both process steam and power, the electrical output may be between 20 and 35 percent, depending on the amount of process steam extracted from the turbine. During this process, a minimum amount of the steam has to pass all the way through the turbine. This means that at least 10 percent of the low pressure steam has to be cooled away. When power and process steam are produced, an extraction turbine is used which may operate as a fully condensing turbine, cooled by seawater or air. When needed, steam can be extracted from a bleed in the turbine at relevant parameters (pressure and temperature). To prevent excessive heat loss and avoid expensive pipelines, the industries that need process steam should be located near the plant.
District Heating Piping Network
District heat is conveyed from production plants to clients as hot water in a closed network consisting of two pipes (flow and return pipes). District heating pipes are laid in the ground, usually at a depth of 0.5 to 1 metre. Pipes consist of a steel pipe, an insulating layer, and an outer casing. The insulating material commonly used is polyurethane foam or similar, while the outer casing is usually high-density polyethylene (HDPE). They can currently expand to around 30km from the generating plant, and distribution networks can be hundreds of kilometres long. The network distance is also easily extended by simply adding more providers of heat, or ‘heat sources’, along the way. On average, heat losses in the distribution network account for less than 10% of the energy transported through the pipes. The return pipe conveys the water back to the production plant for reheating. The temperature of return water from clients to the production plants ranges in best cases between 25 and 50 °C.
