Combined heat and power (CHP) is a process that captures and utilises the heat that is a by-product of the electricity generation process. It contrasts with conventional processes of generating electricity where vast amounts of heat are wasted. In today’s coal and gas fired power stations, up to two thirds of the overall energy consumed is lost, frequently witnessed as a cloud of steam rising from cooling towers.
By generating heat and power simultaneously, CHP can reduce carbon emissions by up to 30% compared to the separate means of conventional generation via a boiler and power station. By using waste heat, CHP plants can reach efficiency ratings in excess of 80%. This compares with the efficiency of gas power stations, which in the UK range between 49% and 52%. Coal-fired plants do even worse with an efficiency of around 38%. For the customer, CHP can provide primary energy savings of approximately 40% compared to the separate purchase of electricity from the national electricity grid and a gas boiler for onsite heating.
A CHP plant is essentially an electrical generator combined with equipment for recovering and using the heat produced by that generator. The generator may be a prime mover such as a gas turbine or a reciprocating engine. Alternatively, it may consist of a steam turbine generating power from high-pressure steam produced in a boiler. In some cases, a CHP scheme may be a combination of prime mover(s), boiler(s) and steam turbine(s).
Combined heat and power plants are typically located close to the end user and thereby help to reduce transportation and distribution losses, improving the overall performance of the electricity transmission and distribution network.
A variety of different fuels can be used to facilitate cogeneration.
Advantages of CHP Packages
The main advantage of the packaged CHP system is that the unit can be manufactured and prepared at the supplier’s premises and then delivered to a site ready for offloading and positioning. Most CHP packages are designed and supplied as complete units, selected to precisely meet the requirements of the site and its energy demands. The package contains a Prime mover which is either a Gas Engine, Small Gas Turbine, or Fuel Cell, the generator and heat recovery equipment, together with all the associated pipework, valves and controls. The equipment is mounted on a steel structure, and surrounded by an enclosure, which reduces noise levels in the adjacent area. The enclosure normally contains a control panel, which is accessible from outside the package.
The preparatory works for installing a CHP package are not complicated, and the main requirement is an area of ground on which the unit can be located. This area must be able to accept the loads of the unit, and must be sufficiently accessible for delivery and positioning. There should always be adequate free space around the package to provide access for maintenance purposes. Furthermore, it is common practice to locate CHP packages within a building to avoid exposure to external weather conditions and provide easy access to services.
A piped supply of gaseous fuel for connection to the CHP package normally requires a pipework connection, which can be either located in a covered access duct within the floor surface or attached to pipe supports and routed at high level within the building. There is also electrical cabling to connect the CHP package to an appropriate part of the site’s electrical distribution system. There is an exhaust system to remove the exhaust gases from the engine or turbine to a point of discharge outside the building area. Access to the site’s hot water supply system is also required as is a suitable means to link to externally located heat rejection equipment.
Most packaged CHP applications supply heat via a hot water connection to a site distribution system, which takes the heat to its point of use. Some applications use an airflow to cool the engine or turbine and this heated air is also then available for use on-site. Furthermore, some units pass their exhaust gases directly to the site for heating purposes, either separately or mixed with a heated airflow. Packages can also be designed to vary their heat outputs between air and water, according to variations in site demand.
CHP systems can have a thermal storage vessel, equipped with a storage control network which can store up to 500 litres. The CHP control system looks to maximise vessel thermal storage capacity. In other words, it keeps the full contents as close to flow temperature as possible. This enables the vessel to make a significant contribution to meeting the peaks in site thermal demand. Other models of CHP systems are designed to allow the mixing of flow and return water in their vessel, which reduces its storage potential.
Financial Benefits of CHP
CHP works by efficiently converting fuel into both electricity and heat on site – improving a building’s energy resilience and reducing costs. With electricity costs increasing, pay back on a CHP system can be achieved in 2-3 years. There is also a reduced usage of relatively expensive back-up gas boilers.
Every hour in operation produces a benefit. By substituting expensive power from the grid with site-generated electricity, the figures speak for themselves: utility companies charge approximately 14p / kWh, whereas gas supplies can be purchased for 5p / kWh.
Environmental Benefits of CHP
As CHP is a highly efficient energy process, it produces significantly fewer combustion products per unit of energy output than traditional discrete heat and power generation systems. This, in turn, has a beneficial effect on air pollution and its consequences. While installing a CHP system usually increases the fuel consumption on-site, the use of a cleaner combustion plant frequently results in a reduction in some of the pollutants produced. In generating electricity on-site, CHP also displaces the larger amounts of fuel which would otherwise be used at central power generating stations, significantly reducing their emissions of pollutants.
CHP Choice & Costs
The capital cost of any CHP plant depends on its size and type. However, there are a number of issues that should be taken into account in relation to costs. Firstly, a CHP plant is more efficient than a simple power plant when the heat output is used effectively. Where CHP power generation produces heat that subsequently remains unused, the plant is effectively operating in the open cycle mode and therefore, probably, at a lower efficiency than the competing external power station.
A plant will operate at its greatest energy efficiency, thereby maximising savings, when it is maintained as close as possible to its maximum load providing all the output is used. Economies of scale do exist, therefore as the size of a CHP plant increases, capital and installation costs, expressed as £/kW, both fall. Operating and maintenance costs are also significant factors, especially for reciprocating-engine-based systems.
Although a plant sized to meet maximum electrical demand will produce the greatest savings in purchased electricity, it may end up operating at part load and thus less efficiently and economically for a greater part of the time.
Although electricity can be exported to the national grid during periods of surplus, these surpluses are most likely to arise at night when selling prices are at their lowest.
Learn More with Therma-Mech
Over the last ten years increasing numbers of establishments have benefited from CHP including office blocks, new housing estates, hotels, hospitals and colleges. Not only have there been significant financial savings resulting from each of these projects, but there has also been a positive environmental gain.
For more information on the benefits of CHP and advice on installing a CHP system, get in touch with us today.