What is CHP?
When dealing with issues of energy supply, one usually thinks about electricity supply although the major energy vector consists of heat instead of electricity. For the Flemish region heat represents 75 to 85% of the total energy demand. Whereas the production of heat and electricity is usually achieved in separate processes, a combined production of heat and electricity in a so called cogeneration process, also known as combined heat and power (CHP) process, offers a lot of advantages, not the least for the environment.
Both heat and electricity are different forms of energy but are not equivalent, since the conversion into another form of energy does not proceed at equal efficiency. To treat the conversion properly, the terms of “exergy” and “anergy” have been introduced. Exergy is that part of the total energy that can be converted completely into another energy vector, while anergy constitutes that part of the total energy that cannot be converted completely into another energy vector. From this definition, it is obvious that “exergy” carries a much larger value than “anergy” and as such merits much more attention in the conversion cycle. Electricity can be converted (almost) completely into other energy vectors such as mechanical energy, heat,…, since it consists of exergy only. Heat is an energy vector of lower quality that cannot be converted completely in other energy vectors. Indeed, heat contains a certain anergy that increases with decreasing temperature of the heat at the detriment of the exergy term.
In most energy production plants, heat and electricity are produced separately in separate installations. As opposed to heat, electricity can easily be transported over long distances and is often generated in large centralized electricity plants located at large distance and connected to the end-users via the transport and distribution networks. Heat on the contrary is mostly produced close to the consumer by means of a boiler or heating system wherein the chemical energy of the fuel is converted to heat via a combustion process. The heat is most frequently used at a low temperature level meaning that the anergy content is large thereby wasting the most noble “exergy” part of the energy content of the fuel.
To avoid such waste of useful energy, combined heat and power (CHP for short) can be the solution. In a CHP installation, electricity and heat are produced simultaneously in a single installation. Since the heat cannot be transported over large distances, the CHP installation is usually located close to the enduser of the heat. The high valued heat (at 1200°C) generated by the combustion of the fuel is primarily used to produce mechanical energy which in turn is converted to electrical energy in a generator. The remaining heat at low temperature (e.g. 500°C) can then be used to satisfy the specific heat demands of a plant, hospital, …
One could say that CHP is a clever way to produce heat with much higher exergetic efficiency by co-generation of electricity. The heat demand is the driving factor and should be used as much as possible. In fact the sizing of a CHP installation should be based on the heat demand. The additional production of electricity is designed to have the least exergetic losses and to produce the heat at the desired temperature. This will ultimately lead to a more rationale use of the energy resources.
There are different technologies available to apply the above principals of cogeneration of heat and electricity. Each technology has its own specific areas for application. The best known technologies are the steam turbine, the gasturbine and the internal combustion engine fuelled either by gas or diesel. Other emerging technologies such as microturbines, Stirling engines or fuel cells are the focus of intense R&D and are on the threshold for commercialization.
The principles of CHP can be enlarged to Trigeneration wherein besides heat and electricity, also cooling is obtained by means of an absorption cooler using the heat as energy input. Such concept can be justified to extend the heat demand also during the summer months and obtain a much flatter heat demand profile all year long. This will lead to a larger load factor and better returns on investment.
The benefit of CHP
The main advantage of CHP is a better utilization of the available energy content in the fuel by cogeneration of useful heat and electricity. This leads inevitably to a reduction of the fuel consumption compared to the separate production of heat and electricity which is highly desirable in the present context of dwindling fossil fuel resources. Most CHP plants use fossil fuels as feedstock, but renewable feedstock such as biomass or biogas is also feasible. This offers the double advantage of using environmental friendly fuel in a most beneficial way.
A reduced consumption of fossil fuels also means a reduction in the emissions of CO2 and other polluting emissions (soot, NOx, SO2, CO,..) which have an important impact on the environment and the climate (Greenhouse effect, Ozone depletion). The KYOTO protocol aims at a reduction of the greenhouse gas emissions, of which CO2 is the most important gas, by 5% compared to the emission level of 1990 to be achieved in the 2008-2012 compliance period. The European Commission targets are even more severe, what for Belgium results in an emission reduction of 7,5% for the greenhouse gases. CHP can contribute in reducing the CO2 emissions but other measures are also needed.
An increase in the number of CHP installations will also mean a shift for the electricity production from strongly centralized generation to more distributed generation. This will also lead to a reduction of the electricity transport losses and reduces the vulnerability of the client upon the availability of a single large generating unit. Hence CHP has the potential to increase the reliability on electricity for the clients.
Rational energy consumption
Rational energy use should contribute towards a sustainable development, i.e. a development which meets the needs of the present without compromising the ability of future generations to meet their own needs. Rational use of energy has to be situated at all levels in the energy chain from energy natural resources, energy conversion to energy consumption. Cogeneration leads to a more efficient conversion of energy and hence contributes to rational energy use. These efforts do make sense only if also the end-user is conscious of the needs to conserve energy.
Primary energy saving by CHP compared to separate production
A company requires a certain amount of heat and electricity and can satisfy this need by either separate production or with a CHP plant. A well designed and properly dimensioned CHP will consume substantially less fuel. Since fuel is primary energy, hence one talks about primary energy savings.
Take as an example a company that needs 45 units of electricity and 50 units of heat to manufacture a product.
One supposes that for separate production, the electrical reference efficiency equals 50% while the reference efficiency for heat production (boiler) equals 90%. It follows that the fuel consumption for separate production amounts to 45/0,5 + 50/0,9 = 146 units.
On the other hand, a CHP plant with a gas motor as driver with an electrical efficiency of 38% and a thermal efficiency of 42% could in theory satisfy the energy demands and would consume 119 energy units of fuel to manufacture the same product. This is 27 units less than with the separate production of heat and electricity, or a relative primary energy saving of 18,5%.
The example above is an ideal case whereby the CHP unit covers exactly the heat and electricity needs simultaneously. In reality this is rarely the case and one must provide the possibility to produce additional energy (heat or electricity) in a conventional way. This will inevitably reduce the primary energy savings.
Units of Energy and Power
Units of Energy
Energy is expressed in the standard-unit Joule (J). Since this turns out to be a very small units, multiples are often used.
1 J (1 Joule)
1 kJ (1 kiloJoule) = 103 J = 1000 J
1 MJ (1 megaJoule) = 106 J = 1 million J
1 GJ (1 gigaJoule) = 109 J = 1 billion J
1 TJ (1 teraJoule) = 1012 J = 1000 billion J
Very often, energy is expressed in kilowatt-hour (kWh) or its multiples. The conversion between both energy units is: 1 kWh = 3600000 J = 3,6 106 J
1 kWh (1 kilowatt-hour) = 3600 kJ
1 MWh (1 megawatt-hour) = 3600 MJ
1 GWh (1 gigawatt-hour) = 3600 GJ
1 TWh (1 terawatt-hour) = 3600 TJ
Units of Power
Power is expressed in Watt (W) and its multiples. Power is defined as energy per unit of time, hence: 1 W = 1 J/s (Joule per second)
1 W (1 Watt)
1 kW (1 kiloWatt) = 1000 W
1 MW (1 megaWatt) = 106 W
1 GW (1 gigaWatt) = 109 W
1 TW (1 teraWatt) = 1012 W
Additional subscripts can be used to indicate what form of energy or power is meant such as electrical, thermal, mechanical,... e.g.:
1 MWe = 1 megawatt electric power
1 MWth = 1 megawatt thermal power
Conversion to primary energy
Usually, the amount of electrical, thermal, ... energy is known. The amount of fuel used to generate that energy can be calculated by dividing the electrical energy by the electrical efficiency, or the thermal energy divided by the thermal efficiency.
Based on conventions, for separate production, the reference electrical efficiency is comprised between 33 and 40%, the reference thermal efficiency is taken at 90%. This means that 1 kWh electricity requires 2.5 à 3 kWh primary energy, and 1 kWh heat corresponds to 1,11 kWh primary energy.