Combined Heat and Power

Introduction

Combined heat and power is an alternative to conventional energy technologies whenever there is a local heat demand and if electricity is needed or can at least be exported to the grid. The underlying principle is to turn the fuel's chemical energy into electricity (high-grade energy) and to use the unavoidable by-product (low-grade thermal energy) for heating purposes.
Small CHP systems rarely provide the exact amount of electricity required, so that they have to be wired through the utility grid which will either absorb the extra electricity output or supply electricity if the output is not sufficient.
The most common way of operating is therefore to run the CHP system at a fixed electrical output which is set by the heat requirements. Indeed, heat cannot be transported and would be wasted if not used on the spot. The electricity produced is then also used locally, with the complement being imported/ exported from/to the grid. This is called base loading¹.
Other operation modes such as peak shaving or emergency stand-by are also possible but are not adapted to the current situation.

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The Various Biomass-Based CHP Technologies

Gas/Diesel Engines
Gas/diesel engines produce shaft power that can be turned into electricity, heat can then be extracted from the hot exhaust gases by means of a heat exchanger. Biomass-based fuels for such technologies include sewage or landfill gas and gas obtained from organic materials through gasification. Bio-oil can also be used and is obtained through pyrolysis of biomass.
The characteristics of usual Diesel Gas/Engines are²:
-Capacity range (kWe): 15-10000
-Electrical efficiency: 30-38%
-Thermal efficiency: 45-50%
-Heat production: 85-100°C
Gas/diesel engines are very common. However they are rarely used when it comes to biomass-based CHP (less than 1% of the current systems installed).

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Micro-Turbines
Here again, gaseous or liquid biomass fuels can be used. The fuel is mixed with compressed air and expands in the turbine, thus providing shaft power and eventually electricity. The thermal energy of the flue gases are usually first exploited in a recuperator which pre-heats the compressed air and then in a conventional heat-exchanger for the heating requirements.
Characteristics of a micro-turbine²:
-Capacity range (kWe): 25-250
-Electrical efficiency: 15-35%
-Thermal efficiency: 50-60%
-Heat production: 85-100°C
There are currently no micro-turbines based on biomass CHP. However, this technology has frequently been used in the United States for other purposes and should soon be mature for biomass applications.

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Steam Engine
In a steam engine, the combustion is external and the flue gases heat the water contained in the boiler. This water then undergoes a Rankine cycle: the steam expands in a turbine, thus generating electricity. It then condenses in the condenser and the heat released is available for local heating.
External combustion means that all kind of biomass fuels can be used: gaseous and liquid as well as solid.
The characteristics of a steam engine are²:
-Capacity range (kWe): 20-1000
-electrical efficiency: 10-20%
-thermal efficiency: 40-70%
-Heat production: 85-120°C
Steam engines are the most widely used technology for CHP biomass-based systems. Many plants ranging from 300kWe to 10 Mwe are currently operating. However, the capacity range below 300 kWe is not yet largely proven.

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Organic Rankine Cycle
Instead of using water as a working fluid, an organic fluid is often preferred when the size of the generating unit is small. This is due to the properties of the organic vapour which are more adapted to low temperature cycles (typically a relatively high saturation pressure is advisable so that low condenser temperatures are attainable without excessive depressurization, besides the saturated vapour line has a positive derivative which excludes condensation in the turbine and thus increases the efficiency of the expansion)³. The high-pressure high-temperature organic vapour from the boiler expands and powers an electric generator via a shaft. The heat released in the condenser is the heating source of the unit and must be at sufficiently high a temperature to suit heating purposes.
The characteristics of an organic Rankine cycle cogeneration unit are²:
-Capacity range (kWe): 200-1500
-Electrical efficiency: 10-20%
-Thermal efficiency: 70-85%
-Heat production: 80-100°C
Such plants are not yet very common but a few have already been installed in Germany, Austria and Switzerland.

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Stirling Engine
Unlike other engines where the combustion takes place within the engine, in a Stirling engine the combustion takes place outside the engine and heat is transferred to the working fluid through a heat exchanger. During the cycle, the working fluid passes through the regenerator, a key component which acts alternatively as a hot or cold thermal store.
Since combustion is external, most biomass combustion technologies can be adapted to Stirling engines. Here again, the heat which has not been converted into work will be recovered in an appropriate heat exchanger for heating purposes.
The characteristics of a small-scale Stirling engine are²:
-Capacity range (kWe): 10-150
-Electrical efficiency (%): 15-35%
-Thermal efficiency (%): 60-80%
-Heat production: 60-80°C
At the moment, this technology is at a pilot stage of development and a few units have been successfully tested in Austria and Denmark.

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CHP Analysis for Barony College

In the case of the Barony college, the heat load is significant in winter (heating and hot water: 270-630kW) and very low in summer (only hot water, 12 kW on a daily average)⁴. The average electric demand for non-heating appliances is estimated at 75 kW throughout the year. The associated problem is that since the heat demand is very low in summer, the electrical power output of the system will also be very low and will only satisfy a very small fraction of the demand. The electrical impact of the CHP system would be negligible.
If the CHP system was to provide a significant fraction of the summer's electrical demand, the heat produced would be mostly wasted and the economic advantage of the CHP system would be lost.
Therefore, it might be advisable to switch off the CHP system during the summer and turn to a back-up boiler for heat and to the grid for electricity.

The fuel available around the Barony college is likely to be woodchips, which means that all the technologies based on gaseous or liquid fuels cannot be considered. This leaves us with the choice of a Stirling Engine, a steam engine or an organic Rankine cycle turbine. As explained previously, Stirling engines are not a very mature technology and are likely to be a very expensive choice. The most likely alternatives are therefore the usual steam engine or the Organic Rankine cycle turbine.

Recommendation: Not viable for Barony College.

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Footnotes

1. The cogeneration sourcebook (F. William Payne)
2. All figures were retrieved from: Open Network: Micro and small-scale CHP from biomass (technology paper 2)
3. Fluid selection for the biomass Organic Rankine Cycle (ORC) in biomass power and heat plants, Ulli Drescher, Dieter Brüggeman, Elsevier
4. Figures calculated on the basis of the Barony college 2005-2006 Energy bills

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References

Industrial boilers (Gunn, Horton)

Power boiler design, Inspection and repair (Mohammad A. Malek)

Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and combustion related environmental issues (Ayhan Demirbas, Progress in energy and combustion sciences, Elsevier)

Experimental studies of a biomass boiler suitable for small district heating systems (Lundgren,Hermansson, Dahl, Biomass and Energy, Elsevier)

Utilizing biomass and waste for power production - a decade of contributing to the understading, interpretation and analysis of deposits and corrosion products (Flemming Jappe Frandsen, Fuel, Elsevier)

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