Ole Elmose, Vibrogegnen's Energy and Environment Office

Decentralized combined heat and power production - cogeneration- is a very flexible and efficient way of utilizing fuels. Cogeneration based on biomass is environmentally friendly, and all kinds of biomass resources can be used.

The role combined heat and power production plays in Danish energy supply originates from the decision in 1978 to establish a national natural gas grid (see chapter 53. At present the natural gas system is one factor blocking the utilization of biomass and natural gas in decentralized cogeneration plants, because a great part of the heat market is lost for decentralized cogeneration due to the individual gas supply.

In June 1986 it was decided that 450 MW decentralized heat and power plants should be established. These are very efficient and environmentally compatible, if they are based on natural gas or biomass. The interest in biomass as basis for combined heat and power production is caused partly by environmental considerations, and partly by the desire in agriculture and forestry to get rid of an increasing surplus of residue products, typically straw and wood chips.

But exceeding the problem with an insufficient heat market, the energy policy has caused that until now there has been no sufficiently purposeful and ambitious aiming at the cogeneration technologies, that first of all shall lead to an increased use of biomass in heat and power supply.

Cogeneration - why ?

For reasons mentioned above and in the chapter on energy planning there is a large political interest in changing the local heat supply to combined heat and power supply - this means cogeneration of heat and power.

It is a fundamental physical condition that not all latent energy of a fuel can be converted into tractive power, e.g. to run a car. The main part of the energy is necessarily transformed to waste heat which in the car example disappears by motor cooling and with the exhaust.

Cogeneration plants can be used in all situations where a given heat demand exists. This include all together an extremely large number of district heating plants, institutions, co-operative building societies, industries, etc.

For the cogeneration technologies described in this chapter the primary interest is due to, that a very large percentage of the fuel's energy content is utilized, typically 85-95%. This must be compared to the relatively low energy efficiency of centralized thermal power plants, the annual mean efficiency is about 55% in the El,SAM area (Jutland, Funen).

Another important reason for the interest in decentralized cogeneration is the possibility to utilize renewable bio fuels straw, wood, manure, etc. There are furthermore a few circumstances which are not that much noticed in the political debate.

First of all a large number of cogeneration plants increase the security of power supply. It is not usual that the large power units break down, but it happens. It is obvious that the consequences of missing a large unit are much more significant, than if it is one of the much smaller cogeneration plants.

Second there is a considerable energy loss from the power grid. In the ELSAM area it is good 7% in average. But this figure covers very large variations through the day, and furthermore depends very much on the voltage level. Thus the energy loss from the low-voltage grid is much larger than from the high-voltage grid. All in all this means that e.g. on a winter day at 5 pm there is a large energy loss from the low-voltage grid.

Exactly because many of the cogeneration plants are coupled on the low-voltage grid, they also reduce the grid loss which influence the overall energy efficiency.

Cogeneration Technologies

Gas Engines

The most common type of combined heat and power production in Denmark is connected to gas-fired internal combustion engines which is a well-known technology. They can be found on the market at sizes from 7 kWpower to about 4 MWpower, and the power efficiency is good 20% for the small engines and over 40% for the largest. As power production is viewed as the main purpose, it is important that the power efficiency is continuously increased.

The lower limit for a profitable cogeneration plant is a heat demand of 15,000 m natural gas per year and a power consumption of 50,000 kWh per year with the current engines at the market.

The gas engine fuel is mainly natural gas and it will remain like this for several years. A few plants are based on biogas which will gain increased utilization, while various types of biogas plants are developed and established.

It is assumed that gas from thermal gasification of straw and wood (see chapter 11) will also spread as fuel for stationary cogeneration plants during the coming years.

There are some differences between cogeneration plants according to operation strategy. The larger plants, typically connected to a district heating plant or an industrial company, are mainly in operation during daytime at weekdays. It is because the payment for power is most favourable at that time, which again is due to that the capacity is paid for during the periods with high consumption. In these cases the cogeneration plant produces heat both for covering the actual consumption and for storage in large water storages. The storages are then emptied for heat at night and during the weekend. It is a political request that 90% of the annual heat consumption must be supplied from the engine, the rest is supplied by a gas boiler.

This operation strategy is only realistic when using natural gas as fuel, as there is enough at a certain time. Contrary to continuously gas production from a biogas or gasification plant. On the other hand the demand for a variable power production will increase, when cogeneration plants with variable production are established.

The smaller plants are typically base load plants that operate day and night. They supply power to own installations and cover the power consumption. In this case the plant has 2 power meters; one that registers buy from the power utility when the consumption exceeds the production, and another that registers sale when own consumption is less than the actual production.

This type of cogeneration plants has severe environmental and resource advantages.

Natural gas is the least polluting of the fossil fuels. It is partly due to the relatively high hydrogen content that becomes water in the combustion. The CO2 emission from natural gas is therefore smaller than from oil and coal.

The NOX pollution from the engines are reduced according to authorities' demand by mounting a 3-way catalyzer or - more often - by using low-NOx engines (lean burn). The smallest engines are excepted from these requirements.

According to resources, the advantage is as already mentioned a higher energy efficiency than at centralized thermal power plants.

Gas Turbines

Some larger district heating plants have based their heat and power production on gas turbines. They can be regulated less than gas engines, and as they by mean of their size presuppose a large heat demand there will not be space for many new in the future. There are simply not that many cities with a sufficiently large heat demand. Apparently there is neither any product development taking place to increase the power efficiency, as it is the case for gas engines.

Combined heat and power production based on steam

The Danish effort to increase the use of biomass mainly straw and wood - as fuel in combined heat and power production, increasingly draws the attention towards steam engines and steam turbines.

The steam engine is a well-known technology, but for different reasons it hasn't been developed for several years. One of the problems has been the contact between lubricating oil and steam. This problem has been solved with a new design of the steam generator which is manufactured in Denmark and is just ready for the market.

The advantage of this cogeneration technology is that biomass can be combusted directly in the steam boiler and obtain the wanted steam pressure of 20-30 bar.

The disadvantage is that power efficiency will hardly exceed 15%. Therefore it is a question if the steam engine is able to compete with cogeneration based on gasified biomass in the longer term.

There seem to be better possibilities for steam turbines with a combination of direct stoking of biomass in the boiler, and superheating of the steam with natural gas. A Danish district heating plant is preparing a test plant based on this technology. Its advantage is a significantly higher power efficiency than the steam engine.

The Stirling Engine

The Stirling engine is a hot-air engine, named after the Scottish priest Stirling who invented it in 1817. Since then it has been designed and manufactured in a vast number of designs.

In spite of intensive and expensive research it is nearly without importance, as the research has been aimed at developing a car engine, which it is not suitable for.

On the other hand there are large perspectives in viewing it as a stationary combined heat and power plant. There is a growing understanding of this that has resulted in new research and production aimed at this. About 150 pieces have been made in batch production in India. This is a simple low-pressure design with a power efficiency of about 10%.

The Stirling engine has many advantages. In principle it is a very simple technology - also in the advanced version with helium instead of air and high mean pressure. Furthermore a big variety of fuels can be used, including concentrated solar heat and clean exhaust from e.g. a gas engine. With the materials used today, it demands about 700›C as optimum working temperature. And the hot air must be that clean, that coating does not occur at the heating surface. Finally, it is nearly noiseless and probably very stable in operation.

The description of the Stirling engine is to a high degree based on the research carried out at the Technical University of Denmark. A 10 kWpower model with helium as medium and a mean pressure of 50 bar has been tested in summer 1992. The results are very promising and it is specially interesting that the power efficiency of this engine is about 30%.

The perspective of heat and power production based on the Stirling engine is that it can probably be produced in a range of 1 kWpower to 150 kWpower in the nearby future; in the longer term maybe with an even higher output.

Such small cogeneration units can give the Stirling engine a tremendous distribution and have a revolutionary influence on our energy supply.