Cooling heating generating power and recovering waste heat with thermoelectric genset

 what is CHP?

Combined Heat and Power (CHP) is the simultaneous generation of usable heat and power (usually electricity) in a single process. CHP is a highly efficient way to use both fossil and renewable fuels and can therefore make a significant contribution to the UK’s sustainable energy goals, bringing environmental, economic and social benefits.

CHP systems can be employed over a wide range of sizes, applications, fuels and technologies. In its simplest form, it employs a gas turbine, an Engine or a steam turbine to drive an alternator, and the resulting electricity can be used either wholly or partially on-site. The heat produced during power generation is recovered, usually in a heat recovery boiler and can be used to raise steam for a number of industrial processes or to provide hot water for space heating.

Because CHP systems make extensive use of the heat produced during the electricity generation process, they can achieve overall efficiencies in excess of 70% at the point of use. In contrast, the efficiency of conventional coal-fired and gas-fired power stations, which discard this heat (via the iconic cooling towers we associate with such installations), is typically around 38% and 48% respectively at the power station. Efficiency at the point of use is lower still because of the losses that occur during transmission and distribution.

In contrast, CHP is a form of a decentralised energy technology. CHP systems are typically installed onsite, supplying customers with heat and power directly at the point of use, therefore helping avoid the significant losses which occur in transmitting electricity from large centralised plant to customers.

What are the benefits of CHP?

CHP delivers a range of economic, environmental and social benefits - some of these accrue to its users, some to the operators of the electricity grid and yet Others to the wider community:

CHP’s high efficiency leads to a reduction in the use of primary energy. Precious fuels are used much more efficiently, so less is used. And less fuel used means significantly lower energy costs to the end user. Savings vary, but can be between 15% and 40% compared to imported electricity and on-site boilers.

Less fuel burnt also means reduced emissions of carbon dioxide (the main greenhouse gas) and other products of combustion. Indeed CHP could provide the largest single contribution to reducing carbon dioxide emissions. Host organisations that wish to reduce their environmental footprint benefit - as well as the environment.

CHP systems can be designed to continue to operate and serve essential loads during an interruption to mains power supplies, increasing security of energy supplies. CHP can also supply higher-quality power than that from the grid - this can be important for computer data centres etc.

Where can CHP be used?

CHP is a family of energy conversion processes, rather than a single technology, so it can be used to provide energy to anything from a single home to a large industrial plant, or even a whole city.

Unlike conventional power plants, CHP units are sited close to where their energy output is to be used.

The main design criterion is that, to make the investment worthwhile, there must be a need for both the heat/cooling and electricity produced by the CHP unit.

In the home, a MicroCHP unit resembling a gas-fired boiler will provide both heat for space and water heating, as does a boiler, but also electricity to power domestic lights and appliances. MicroCHP units are a very new technology only recently appearing in the UK market, but the potential for them is as large as the number of homes in the country.

For commercial buildings and small industrial spaces, a factory-assembled, ‘packaged’ CHP system is appropriate. Here, an electricity Generator, heat exchanger, controls and either an engine or a turbine is packaged together into a CHP unit that can be connected to the heating and electricity systems of the building.

Some building types, particularly those that need a lot of energy, or operate around the clock, are particularly suitable for CHP - leisure centres, hotels, hospitals and many others. CHP systems can, with the addition of a chiller, supply cooling for air conditioning systems as well as heating - such an arrangement is often called a ‘trigeneration’ system.

Homes and buildings fitted with CHP are usually also connected to the mains electricity grid, and may also retain back-up boilers, so that they are never short of an energy supply, during maintenance of the CHP plant, for example, or during periods of unusually-high energy loads.

Industrial CHP plants tend to be designed and built individually to fit the industrial process they serve. These CHP plants are based on gas turbines, steam turbines or engines, together with electricity generators and control systems. The very largest CHP plants rival traditional power-only plants in size and deliver huge quantities of energy - but at a much higher efficiency Some industrial processes are particularly well-suited to CHP, those that use lots of heat and operate around the clock - the manufacture of paper, chemicals, food and drink products, as well as refineries, are among those that can benefit most from CHP.

Community heating systems serve whole towns, areas of cities or, in a few cases, whole cities. Here, one or more CHP plants supply heating to a grid of insulated hot water pipes that carry heat to a range of buildings, including public and private sector flats. As well as CHP plants, boilers and other sources of heat may feed heat into the grid. Buildings that take heat from the community heating system do not need their own boilers. Meanwhile, the electricity generated is used to help run the community heating plant, and within the customer buildings, or is exported to the electricity grid.

Biomass & CHP

Our main interest is in the use of Biomass to power CHP schemes. The newest method for energy generation is known as gasification. This method captures 65-70% of the energy present in solid fuels by first converting it into combustible gases. These gases are then burned, just as we currently burn natural gas to create energy.

When biomass is heated with no oxygen or only about one-third the oxygen needed for efficient combustion (amount of oxygen and other conditions determine if biomass gasifies or pyrolyzes), it gasifies to a mixture of carbon monoxide and hydrogen – synthesis gas or syngas.

Combustion is a function of the mixture of oxygen with the hydrocarbon fuel. Gaseous fuels mix with oxygen more easily than liquid fuels, which in turn mix more easily than solid fuels. Syngas therefore inherently burns more efficiently and cleanly than the solid biomass from which it was made. Biomass gasification can thus improve the efficiency of large-scale biomass power facilities such as those for forest industry residues and specialized facilities such as black liquor recovery boilers of the pulp and paper industry – both major sources of biomass power. Like natural gas, syngas can also be burned in gas turbines, a more efficient electrical generation technology than steam boilers to which solid biomass and fossil fuels are limited.

Most electrical generation systems are relatively inefficient, losing half to two-thirds of the energy as waste heat. If that heat can be used for an industrial process, space heating, or another purpose, efficiency can be greatly increased. Small modular bio-power systems are more easily used for such "cogeneration" than most large-scale electrical generation.

This technology uses Vertical Integrated Gasification Combustion (VIGC) units. These units use carefully designed internal chambers and thermal reflectivity. Ensuring balanced air/fuel throughput, complete combustion is achieved making it a very clean process.

Power output is achieved between 25kW to 1MW per single module (up to 5 modules are available) through one of three electricity generating technologies.

What waste cannot be used?