The development of micro-CHP technology is well advanced in the Netherlands, where field trials of several devices are either underway or planned for later this year. Hans Overdiep reviews progress made so far and considers the future patterns of use for domestic-scale CHP.

In the Netherlands, electricity generation is already relatively clean – mainly thanks to the widespread use of combined heat and power (CHP) in the industrial and horticultural sectors. CHP appliances make efficient use of the heat released through electricity generation. Because CHP plants run mainly on natural gas (the cleanest of all fossil fuels), they are seen as a relatively clean way of generating electricity (low level of emissions, high yield). The share of sustainably produced electricity is also growing, mainly through the large-scale use of wind power and the consumption of non-fossil fuels by power stations.

The efficiency of electricity generation can be further improved through the application of smaller CHP plants in the built environment, for example, in the form of mini-CHPs in the utilities sector and micro-CHPs in the domestic market. The development of the latter (otherwise known as the micro-CHP boiler or home power plant) is now well under way.

The home power plant is widely regarded as one of the most promising successors to the high efficiency boiler, which has been on the Dutch market for around 25 years and can be seen as the standard to heat homes. The home power plant will be marketed at the end of 2009 or the beginning of 2010 by one or two of the larger boiler manufacturers in the Netherlands.

Micro-CHP (home power plant)

The original high efficiency boiler, which was developed by Gasunie (the main Dutch utility and a major European gas company) around 1980, has saved billions of cubic metres of natural gas over the past few decades. It has been developed further by various manufacturers to produce the standard now seen in millions of households in the Netherlands. High efficiency boilers achieve a very high score as a safe, reliable, energy efficient and comfortable supplier of heat. In recent years there have been a number of successful attempts to boost their output of hot water. However, such appliances are now at the end of their development possibilities. Promising technology that could provide a successor to the high-efficiency boiler includes:

  • heat pump (gas and electrically powered)
  • hybrid boiler (combination of a high efficiency boiler and a small electric heat pump)
  • solar boiler combi
  • micro-CHP.

Interest in gas-fired micro-CHP boilers is considerable. The ability to generate both heat and power efficiently through a high efficiency domestic boiler is clearly an attractive proposition. Micro-CHPs that satisfy certain minimum requirements governing heat and electricity output are referred to as ‘home power plants’. These minimum requirements were launched by the Dutch Smart Power Foundation (, which has registered them with the Energy Performance Inspectorate of the Stichting Energie Prestatie Keur (Stichting EPK) – an independent organization that promotes the use of, and issues quality labels for, low-energy, clean and efficient central heating, hot water and other installation appliances relevant to sustainable energy.

As early as 1994, Gasunie began studying the possibilities of developing micro-CHP into a viable successor to the high efficiency boiler (part of Gasunie’s mandate is to promote the efficient use of natural gas). Following the demerger of N.V. Nederlandse Gasunie into two separate companies – Gasunie (Transport) and Gasunie Trade & Supply (Trading) – this task was taken on by Gasunie Trade & Supply (now GasTerra;

By 2004, a number of different micro-CHP systems had been tested and installed in domestic settings. However, the experience gained with these systems was not thought promising enough to justify a more extensive field trial with these appliances. The overall efficiency of the tested micro-CHP units driven by Stirling engines was too low to achieve a specified saving in fossil fuel when compared to the standard high efficiency boiler as a reference.

Initial field trial

In 2004, however, initial experience with an appliance from New Zealand, the WhisperGen Mk4 (supplied by Whisper Tech Ltd; generated enough confidence to prompt a field trial of several appliances in collaboration with a number of market players. The WhisperGen Mk4 delivers an electric output of approximately 1 kW and a thermal output of approximately 7 kW, and is based on a four-piston Stirling engine.

Whispergen mark 4 unit with a 200 litre storage vessel
Click here to enlarge image

In spring 2005, GasTerra initiated a field trial with 50 micro-CHPs in practical test situations. These were mainly at the homes of people working for the energy companies, Stichting Natuur en Milieu (Netherlands Society for Nature and Environment) and on cogeneration projects. The field trial ended in spring 2007. The aims were to:

  • gain initial experience in the production of domestic household electricity using micro-CHPs
  • raise awareness of micro-CHPs
  • promote discussion about micro-CHPs including whether it was possible to resupply electricity to the grid (introduction of a capacity tariff to replace the transport tariff, double VAT/energy tax, installing a smart E-meter).

The 50 WhisperGen units were combined with an indirectly fired hot water storage device; the existing combi-boilers in the test homes were left in place since it was thought that the Mk4 would have too low a thermal output to maintain the temperature throughout the dwellings in colder weather. Initial results with the 50 units were positive. It was concluded that:

  • micro-CHPs would be easy to install in Dutch households
  • generating domestic electricity using natural gas would be viable.

Discussions about the micro-CHP were duly set in motion. However, no fossil fuel savings were found in the field trial.

Recent developments

In 2005 GasTerra concluded agreements with the UK company Microgen ( to demonstrate prototypes of micro-CHP combi appliances based on the model developed by Microgen and which would be suitable for the Dutch market.

Microgen unit with 100 litre storage vessel
Click here to enlarge image

Microgen’s micro-CHP is a free-piston Stirling engine with an electrical output of approximately 1 kW. The partnership focused mainly on the development of a system for heating tap water as a supplement to Microgen’s own micro-CHP design. A combination of this kind has since been satisfactorily tested in a field trial in the northern Netherlands.

At the ISH 2007 trade exhibition, the Dutch boiler maker Remeha ( announced that it intended to market a compact wall-mounted home power plant boiler (as a combi boiler) incorporating Microgen’s micro-CHP technology by the end of 2008. Combining the Microgen Stirling engine with a compact high-efficiency boiler module and a compact plate exchanger would create a home power plant combi boiler with sufficient output to heat larger homes and to provide a comfortable supply of hot water. Other major boiler makers in Europe have also begun developing the home power plant (see also

Remeha combi micro-CHP unit
Click here to enlarge image

In 2007, GasTerra together with the energy companies Eneco, Essent and Nuon initiated a first field trial with eight Remeha combi micro-CHP units. Another 100 of these units were placed in homes during 2008 as a bigger field trial in the Netherlands.

Other boiler makers such as Vaillant, Viessmann and Baxi are already demonstrating prototypes of a Microgen Stirling engine combined with a small central heating boiler in a compact encasing. Field trials with Vaillant and Baxi are planned for 2008.

Another supplier of the free-piston Stirling engine is the Dutch company Enatec ( This engine delivers approximately 1 kW of electricity and will eventually find its way into many central heating boilers. Boiler makers such as Bosch, Nefit and Merloni will use this engine in their micro-CHP units. Field trials with those three boiler makers are planned for 2008.

As well as the Stirling engine, work is also under way on various fronts to develop micro-CHP technology based on the organic Rankine cycle (ORC) concept, gas engines, gas turbines and fuel cells. The Dutch firm Daalderop already has a prototype of an ORC-driven micro-CHP. A field trial with several units is planned for 2008.

Whisper Tech Ltd recently launched the WhisperGen Mk5 as a successor to the WhisperGen Mk4. This appliance has an additional burner which means that, if it is combined with an indirectly fired hot water storage vessel, it can – unlike the Mk4 – replace a combi boiler in a large home. The Mk5 has a thermal capacity of approximately 12 kW, enough for a domestic household with a gas consumption of up to approximately 2500 m3/year. Hot water can be comfortably obtained through the installation of an indirectly fired 120-litre storage vessel.

A micro-CHP should ideally have a storage system for hot water. The micro-CHP unit (e.g. the Stirling engine) is not suitable for producing instantaneous hot water because it takes too long to heat up. The engine does not start until the hot water tap has been on for a few minutes and, no sooner does it begin to produce electricity than the tap is turned off again. Preference for heating tap water is therefore given to an indirectly fired boiler. However, the new micro-CHP from Whisper Tech can stay in operation long enough to generate electricity as well as satisfy demand for hot water. This means that electricity can also be produced during the summer.

Research is under way on compact heat storage systems based on phase transition materials. It is hoped that this will give an indirectly fired 100-litre storage system the heat capacity of one twice that size.

An indirectly fired hot water storage vessel combined with a micro-CHP means the system could be expanded with solar panels in the future to make the most of heat generated from renewable energy (especially in the summer). Supplementing the system with photovoltaic panels for the sustainable production of electricity completes the whole. In the summer, both heat and electricity could be sustainably generated from panels on the roof. The micro-CHP would supplement this power when required.

To maximize the output of the micro-CHP, the heat required by the boiler and the dwelling should be met as far as possible by the micro-CHP unit. The high efficiency boiler module or supplementary burner should be used as little as possible. However, it is important to cater for comfort requirements; if the dwelling and the boiler take too long to heat up, this will lead to discomfort. Applying knowledge of the ‘cascade’ scheme for boiler installations and (hybrid) heat pumps is advised here.

Energy conservation potential and market opportunities

Because a high-efficiency home power plant (i.e. a micro-CHP that satisfies the minimum requirements) generates electricity whenever there is a demand for heat (from the boiler or the home) and the combustion gases of the micro-CHP technology in the appliance condense, there is a supply of ‘high efficiency heat’ and ‘high efficiency electricity’. The generation efficiency of electricity in the home is thus approximately twice as high as current ‘mains supply’ efficiency (efficiency of the existing central grid minus the transport losses; carbon dioxide (CO2) emissions from a home power plant are approximately 190 g/kWh). This is where the savings potential of the home power plant lies – both in terms of CO2 emissions and fossil fuel combustion.

The launch of the first home power plants capable of generating approximately 2500 kWh in the home using approximately 275 m3 of natural gas are expected in the near future. Generating this 2500 kWh from the central grid and transporting it to the home would require at least 550 m3 of natural gas (equivalents). So in terms of reducing CO2 emissions, replacing a high efficiency boiler with a home power plant will save up to 1000 kg of CO2 per home per year.

The Smart Power Foundation (SPF) has calculated the market potential of the home power plant for 2020 and 2030. In 2020, SPF expects there to be over 1.5 million appliances on the market in the Netherlands and predicts that, by 2030, this number could rise to over 4 million. Based on this forecast, the savings potential in 2030 would be 10%–18% in the built environment (compared with 2005) and the reduction in CO2 emissions would be 10%–33%.

Those manufacturers currently working on the development of the home power plant have indicated that its additional cost compared with the high efficiency combi boiler could be recovered in approximately five years. When it was first developed, the high efficiency boiler was awarded a government subsidy and this worked well. A similar incentive is now needed for the home power plant.

If the home power plant generates more electricity than can be used in the home, it is desirable for the surplus to be supplied to the grid. But to guarantee an acceptable cost recovery period, it will be necessary for the ‘resupply’ price for this electricity to be comparable to the price paid for the electricity by the end user. Both the Dutch government and Dutch energy companies are working on an acceptable resupply price designed to increase the home power plant’s chances of success. In the near future, the transport tariff for the supply of electricity to domestic consumers is likely to be replaced by a capacity tariff (connection fee). If there is no adjustment of the energy tax, this capacity tariff could have a more damaging effect on the cost recovery period for micro-CHPs than the transport tariff.


For the foreseeable future, the home power plant will operate only in response to demand for heating. However, it is also possible for the appliance to start up in response to demand for electricity. This could happen if, for example, an imbalance in the grid gave rise to a local demand for electricity. Using home power plants wherever possible to cover demand during the relatively expensive peak demand period in the morning and evening could also be an attractive proposition. There are examples in which home power plants can be operated remotely in response to demand for electricity. Obviously, the heat generated should be stored as efficiently as possible for later use.

Remote control and efficient use of local electricity generators (different forms of cogeneration, sustainably produced electricity using solar and wind power) is also referred to as a virtual power plant (VPP). Initial experiments with small-scale VPPs are now taking place (see also

Home power plants could also acquire yet another role in the future. If in time a large-scale electricity generator were to be built, say, in the form of an offshore wind farm, it is conceivable that during times of high wind, a surplus of electricity could be produced during the night. One possibility might be to convert this electricity in households (through electrolysis or a reversible fuel cell) and to store the hydrogen released either in the natural gas delivery networks or in a separate hydrogen network, or even in a domestic storage system (enabling future consumers to refuel their hydrogen-powered cars at home). The heat released during electrolysis could then be stored. At a later stage, for example in the morning, the hydrogen could be used to generate electricity and heat. This could be the best way to utilize the supply of large-scale sustainable electricity production. By then, the home power plant will presumably already have been producing heat and power more sustainably following the mixing of natural gas with gas generated from biomass or sustainably produced hydrogen. This would constitute a very satisfactory transition phase from fossil fuels to renewable sources.

Over the coming years, GasTerra will work with energy suppliers (e.g. Eneco, Essent and Nuon) on the large-scale application of small local electricity generators such as the home power plant. These companies have joined together to establish Stichting Slim met Gas (Foundation Smart with Gas;, which is organizing a field trial/demonstration project with 10,000 home power plants during the period 2007–2010.

History thus appears to be repeating itself: just as the high efficiency boiler has saved billions of cubic metres of natural gas in decades gone by, its potential successor, the home power plant, could save many billions more if it is rolled out on a large scale.

Hans Overdiep is with GasTerra, previously Gasunie Trade & Supply, Groningen, the Netherlands.