Principle and advantages of cogeneration
Cogeneration means the combined production of electricity and heat.
From the physical nature of the Carnot cycle, classical independent electricity production is accompanied by energy losses in the form of low-potential heat. In the case of cogeneration, this residual heat is used, for example, for heating or technology, thus saving part of the primary fuel.
Cogeneration as a principle is also applied in small scales of cogeneration units with gas engines.
It is actually a CHP unit (Blockheizkraftwerk BHKW) burning gas in a piston engine. The input energy in the fuel is transformed into electrical and thermal energy in the cogeneration unit. We drive an electric generator (today almost exclusively synchronous) with mechanical energy. However, the residual heat from engine and exhaust cooling is further usefully utilized.
In addition to its output, important ratios of the cogeneration unit are also the ratio between the electricity produced and heat.
Related to this are the most monitored efficiencies today (electrical and thermal) and pollutant emissions.
However, the most important parameter is and always remains the reliability of operation.
Reliability must then be supported by a functioning service.
The requirements of customers and regulators are aimed at cogeneration to maximize efficiency while minimizing pollutant emissions.
As usual in these cases, these requirements are conflicting and the compromise comes at a price.
The price is expressed by higher investment and operating costs, which are offset by fuel savings and less damage to the environment.
In the field of climate change protection, burning natural gas is more favorable than burning coal, but of course it is not neutrality.
From the point of view of climate change protection, cogeneration burning renewable fuels is significantly better.
The use of sewage gas from wastewater treatment plants, biogas plants disposing of agricultural or municipal waste or landfill gases is particularly deserving.
Imagine a landfill without active degassing.
Where is the next of technological development of cogeneration units?
In the area of emission regulations for nitrogen oxides, unfortunately, so far very expensive solutions SCR (selective catalytic reduction) for supercharged engines of medium and higher power. This technology is not only investment but also space consuming. In addition, it requires the addition of urea needed for the chemical reduction of nitrogen oxides in the exhaust gas. You know this solution in a small diesel engines of passenger cars under the name AdBlue. There is an easier way to reduce nitrogen oxide emissions by using a three-way controlled catalytic converter. It is a relatively simple solution, space-saving, without the need for additional chemistry. Unfortunately, it is practically only available for machines with smaller unit outputs.
As for the improvement of electrical efficiency, it takes place under the direction of gas engine manufacturers in the area of improving the combustion process in engines. Today, engines are at a very high level of development and therefore higher operating pressures are needed, which are often reflected in high sensitivity to changing conditions. Sophisticated control systems regulating current parameters at the limit of the engine’s technical capabilities have become a matter of course. Such machines tend to be more delicate and it is often questionable whether the savings in efficiency are thwarted by higher service costs for keeping the technology in operation. Today, modern gas engines achieve electrical efficiencies in the range of 37 to 43%, depending on their size. Manufacturers of cogeneration units can usually find a few tenths of a percent of the electrical efficiency when optimizing the efficiency when optimizing the size and type of generator.
Another priority is the optimization of heat production. The largest reserve is in the use of heat from exhaust gas. Increasingly, a two-stage exhaust gas exchanger is sometimes proposed (economizer or condenser). Thermal efficiency at the limit of technological possibilities of cogeneration is today around 50%.
As a rule, peak values of total efficiencies (electrical + thermal) today reach 90% or slightly above. Smaller outputs have lower electrical and higher thermal efficiency, with higher outputs the opposite is true.
Example where used two stage exhaust gas heat exchager in Chotebor Czechia – 854 kWe / 946 kWt
Electrical efficiency 42,8% | Thermal efficiency 48% | Total efficiency 90,8%
- stage EGHE 452 kWt (443 > 120 oC)
- stage Economizer 49 kWt (120 > 82 oC)
Probably the biggest challenge for developers of piston engines of all kinds is the safe and reliable combustion of hydrogen. All manufacturers are working hard on this issue and some are starting to present the results of the development on pilot projects. I am not convinced that we really have an effective solution for burning pure hydrogen in cogeneration units. However, it is very likely that it will soon be possible to safely and economically burn a mixture of natural gas and hydrogen in a flexible concentration of 0 to 20% in reciprocating engines. Such a technology would make it possible to significantly increase the accumulation of surplus renewable energy in hydrogen and its back conversion in CHP.
The production of electricity from gas, and preferably cogeneration, is the most suitable complement to renewable sources. It is a source that is ideally flexible and independent of the weather. This is a crucial solution, especially in the heating plant and for industrial use. You may disagree with that, but that’s all you can do about it.