REHVA Journal – December 2011

Klaus Sommer,Ph.D
Professor at Cologne University of Applied Sciences


With the transition towards energy supply which has to be based more and more on energy efficiency and CO2 reduction the decentralized energy systems will play an important role now and in the future. In general the combined heat and power (CHP) technology is a well-known and established engineering solution for energy efficient energy supply. But it is new that the market is getting ready now also for small-sized CHP units for one- or two-family houses as it was presented at this year´s ISH in Frankfurt/Germany for example under the topic of Micro-CHP appliances. This article describes these appliances only for the three different technologies “Stirling Engine”, “Internal Combustion Engine” and “Fuel Cell” with the help of some product examples.

Stirling Engine

The ViessmannVitotwin 300-W (Figure 1) is an example for a Micro-CHP appliance with a Stirling engine (Figure 2) which is hermetically sealed and almost maintenance-free. It runs quietly and therefore it allows an installation close to the living space.

The gas fired stirling engine generates 6 kW heat with an efficiency of 81% (gross cv) and 1 kW power with an efficiency of 15% (gross cv) simultaneously. The integrated gas fired peak load boiler of the Vitotwin 300-W can generate heat between 6 kW and 20 kW in addition with 98% efficiency (gross cv). Therefore this high-efficient Micro-CHP is suitable for old and new detached houses and also for two-family houses. With an annual consumption of gas of 26 000 kWh or more and power consumption above 3 000 kWh per year the Micro-CHP works with exceptional economy.

The stirling engine operates in a heat-led mode. The constantly generated power of 1 kW covers the base load of the electricity demand on site and surplus power is exported to the public grid, which is remunerated.

Due to the daily fluctuation of the heat consumption on the one hand and the constantly generated heat during the operation of the Micro-CHP on the other hand  the combination with a heating water buffer cylinder is essential (Figure 3).

1 - ViesmannInox-Radial heat exchanger η= 96%

2 - MatriX cylinder burner, Modulation range 4.5–20 kW, Nox <40 mg/kWh, CO <50 mg/kWh

3 - Air distribution valve

4 - Stirling burner

5 - Stirling engine, Performance = 1 kWel, Maintenance-free

6 - Control

Figure 1. Wall mounted ViessmannVitotwin 300-W Micro-CHP appliance with a Stirling engine (lower half) and with integral peak load boiler (upper half). Image source:

A - Hot area, B - Displacer piston, C - Cold area, D - Working piston, E  - Power generation, F - Cylinder retriever

Figure 2. Hermetically sealed Stirling engine with the heating-up heat exchanger and the displacer in the upper part, water-cooled heat exchanger in the middle part and power piston, springs, coil and  magnets (Linear Generator for electrical output) in the lower part. The displacer moves helium from the heated area to the cooled area. The alternately heated and cooled helium expands and contracts what creates pressure waves and that then again let the working piston move. The working piston is a part of the linear generator which produces electricity while the working piston is moving. Image source:


Figure 3.ViessmannVitotwin 300-W Micro-CHP appliance with a heating water buffer cylinder. Image source:


Internal Combustion Engine

The VaillantecoPOWER 1.0 is an example of a Micro-CHP appliance with an internal combustion engine (Figure 6).

Figure 4. Gas-Internal Combustion Engine (single-cylinder four-stroke piston engine). Electrical output 1 kW with efficiency of 26.3%, thermal output 2.5 kW with efficiency of 65.7% and total efficiency of 92% (nettcv). Image source:

Figure 5 demonstrates all the components which belong to the VaillantecoPOWER 1.0 system. It operates in a heat-led mode and is suitable for single-family and two-family houses with an annual heat demand of 15 000 to 25 000 kWh per year. Together with the different available peak load boilers the Micro-CHP system can generate a thermal output of 2.5 to 28.3 kW (60/40°C). With an annual operation above 4 500 hours per year the Micro-CHP works with exceptional economy.

Figure 5. Components of the Micro-CHP system of VaillantecoPower 1.0: wall-mounted gas-fired peak-load condensing boiler (left), Micro-CHP unit and heat-transfer module with system controller (center), multi-function cylinder with 300 liter or 500 liter capacity and external heat exchanger for domestic hot water (right). Image source:

In Figure 6 two scenarios of heat and power supply of a building are compared. On the one hand there is a power generation with a coal-fired power plant together with a gas boiler on site and on the other hand there is a micro-CHP on site. In this example it turns out that the micro-CHP appliance can save 30% of the primary energy compared with the low-efficient power plant solution.

Figure 6.Comparison between a separated and a micro-combined heat and power generation. Image source:


Fuel Cell based micro-CHP unit

A product example of a fuel cell-based micro-CHP unit called “BlueGen” is shown in Figure 7. The natural gas-fired unit works with a high temperature solid oxide fuel cell (Figures 8 and 10) and can generate up to 2 kW electrical output with up to 60% efficiency (nettcv). Then with the utilization of the simultaneously generated thermal output up to 1 kW W the total efficiency of the “BlueGen” can increase above 80%. For this case of operation the CO2-reduction could be 50% and more compared with a separated heat and power generation of average existing energy efficiency.

Ongoing tests just try to find out how far the “BlueGen” can cover the annual heat consumption and also the annual electrical consumption of single-family or two-family houses in a power-led mode with a peak load boiler and an optimized heating water buffer cylinder in addition (1).


Figure 7. In front with about the size of a washing machine the fuel cell-based micro-CHP unit “BlueGen” of Ceremic Fuel Cells Ltd. together with a different heating water test buffer cylinder (1). Laboratory of Heating Technology at Cologne University of Applied Sciences/Germany ( Image source:


Figure 8. Functional principle of the high temperature Solid Oxide Fuel Cell (SOFC); source: Ceramic Fuel Cells Limited. Image source:


Figure 9.Opened “BlueGen” with the high temperature solid oxide fuel cell stack in the upper part.Laboratory of Heating Technology at Cologne University of Applied Sciences/Germany (


Compared with separately generated heat and power the gas-fired micro-CHP appliances with stirling engine, internal combustion engine or high temperature solid oxide fuel cell save energy and reduce CO2-production for old and new one- or two-family houses. The heat-led “Stirling engine” and “Internal combustion engine” are designed for 1 kW electrical output with an electrical efficiency of 15% and 26.3% and CHP-generated thermal output of 2.5 kW or 6 kW. Together with a peak load boiler the thermal output of these appliances varies up to around 28 kW dependent on the chosen performance of the peak load boiler. The high temperature solid oxide fuel cell is designed for power-led operation with a very high electrical efficiency up to 60% and an electrical output up to 2 kW together with a thermal output up to 1 kW.


(1) Two-year research project (2011-2013) “Analyzing of the real operating characteristics of a high-efficient fuel Cell-based Micro-CHP in order to find out the best possible application for residential buildings and to get real system characteristics”; project leader Klaus Sommer. Funded by KlimakreisKoeln (, Rheinenergie ( and Cologne University of Applied Sciences (

Klaus Sommer,Ph.DPage

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