Combined heat and power unit: generate your own electricity and heat

large block heat and power plant also exists in small for real estate

A block heat and power plant generates electricity and heat at the same time and is, depending on the size, also interesting for single-family houses. How a CHP unit works, what fuels it can run on, and what you should know about costs and financial subsidies.

What is a cogeneration unit and how does it work?

A combined heat and power plant (CHP) uses cogeneration to generate electricity and heat for heating and hot water from a single fuel. Due to the double benefit you have an extremely high efficiency. The power plants are available both in large form for supplying entire city districts and as compact, mini, micro or nano CHP units for private single and multi-family homes.

Good to know: efficiency describes the efficiency of a system, in concrete terms the ratio between energy used and usable energy.

The core of a CHP unit's operation is a combustion engine. Unlike a car, however, the energy generated by burning a fuel is not used for acceleration on the road, but to drive a generator to produce electricity. The heat generated during combustion is used to operate the heating system and to heat water. Electricity and heat are generated in a ratio of approximately 1:3 – one kilowatt of electrical power provides about three kilowatts of thermal power for heating.

What types of CHP are there?

On the one hand, cogeneration units are classified according to the amount of electrical power they produce and are accordingly suitable for different objects:

  • Combined heat and power plants from 5 mega-watts for entire neighborhoods
  • (large-scale) CHP units with 50 kw to 5 MW for residential blocks, industry and public buildings
  • Mini CHP units with 15 to 50 kw for smaller commercial premises and apartment buildings
  • Micro-CHP with 2.5 to 15 kw for single- to multi-family homes
  • Nano CHP units up to 2.5 kw for one- to two-family homes

The smallest cogeneration units are still relatively new on the market, but they can cover the basic needs for electricity, heating and hot water for a single-family home quite well.

Fuels and technologies for the combined heat and power plant

A second distinguishing feature of CHP units is the fuel that is used.

CHP with gas

Here, the operation is similar to a diesel car: the engine, which runs on natural gas, converts chemical energy into mechanical energy and generates electricity through a generator. The heat generated is used for heating and hot water. Since gas heating is widespread and therefore a gas connection is available in many places, a cogeneration unit powered by gas is one of the most commonly found devices. You can decide whether you want to use gas from fossil sources or from renewable sources. In both cases, the combustion is considered clean and the efficiency of the CHP high.

Tip: if you don't have a gas connection in your house, you can also run a combined heat and power unit with liquid gas. However, this requires additional tanks to store the gas.

CHP with fuel oil

If you (still) own an oil-fired heating system, you can supplement it with an oil-fired CHP unit. Due to the relatively high cost and storage requirements, a complete switch to running on oil is not worth it – especially since there are now much more environmentally friendly alternatives available.

Pellet cogeneration plant

Here, low-cost and carbon-neutral wood pressings are burned to generate electricity and heat. With the right technology, heating efficiency of up to 90 percent can be achieved in this process.

Speaking of technology, there are differences here as well.

Here's how a stirling engine converts heat from an external energy source into mechanical energy. A gas is repeatedly heated and cooled in an enclosed space – the volume of the gas varies, driving a power-generating piston. Residual and waste heat are used for heating. The efficiency of electricity generation here is "only" 12 to 15 percent – for heat generation it is a very good 80 to 90 percent.

An alternative is a fuel cell heater in which water and oxygen react in a controlled manner to produce electricity and heat. Since the efficiency of electricity generation is significantly higher at up to 40 percent, this technology is particularly interesting for properties where the heat demand is less pronounced – after all, the ratio between generated electricity and heat is also 1:3 here.

(when) is a combined heat and power unit for single-family homes worthwhile??

As a rule, a combined heat and power plant operates heat-led. Means: only when heat is needed in the house, the CHP unit also works and generates heat and electricity. If no heat is needed, no electricity is produced. The base load is set as the base value for the operation, i.E. The heat demand over large parts of the year.

Tip: to ensure that sufficient heat and hot water is also available in winter, the heating system should be combined with a peak-load boiler.

For a cogeneration plant to be worthwhile for a single-family home, the household ideally consumes exactly the amount of heat produced by the cogeneration plant itself. The electricity produced should also be used first and foremost by yourself – this way you can easily save 30 cents or more per kilowatt hour of electricity that you would otherwise have to pay to the utility company. If there is still electricity left over, you can feed it into the public grid – for this you get a payment of 10 to 12 cents. If they settle these savings and if necessary incomes with the purchase price, it becomes fast clear that finally above all a long running time of the own bhkws is crucial for the profitability.

Cost of a combined heat and power plant

If you decide to use a gas-fired CHP unit and do not yet have a gas connection in your house, you must first take care of the connection to the public supply network. Laying a gas pipe from the street to your house can quickly cost up to 3.000 euro cost. Similar costs are incurred for the installation of tanks for the supply of liquid gas.

The cogeneration unit itself costs between 14.000 and 17.000 euro and consists of the CHP unit, a peak load boiler already recommended above, and a storage tank. The latter absorbs surplus heat generated.

Costs for fuel are incurred during operation. However, these are often offset by the savings in external electricity costs and, if applicable, the remuneration for electricity fed into the grid.

Promotion of combined heat and power plants

Who decides only now for the acquisition of own bhkws, misses unfortunately a large national promotion: the federal office for economics and export control (BAFA) subsidized up to end of 2020 the installation of a BHKW, depending upon achievement, with several thousand euro.

However, alternative funding options are offered by kfw: on the one hand, low-interest loans are granted for the purchase of the plant (kfw program 270), if it runs on the basis of solid biomass, biogas or geothermal energy. On the other hand, fuel cells can be subsidized through kfw program 433 – up to 40 percent of the costs are covered.

If you live in a house that is at least 10 years old, there is also the tax bonus for renovations since 2020: property owners can claim 20 percent of renovation costs against tax over 3 years. This may include the initial cost of a cogeneration unit.