Re-using waste heat

Paul Boughton

Jeroen van Ruitenbeek and Johan van der Kamp look at the efficient use of waste heat in combined heat and power systems.

Generating electricity, a (fossil) fuel - gas, coal, or oil - is converted into heat and subsequently is being used to generate high pressure, high temperature steam. Then, in a steam turbine connected to a generator, a significant part of the energy in this steam is converted into electrical energy to be supplied to the power grid.

In large modern power stations, approximately 40 per cent of the energy in fossil fuel is converted into electrical energy, the remainder being discharged in (cooling) water as waste heat, and/or into the air. That waste energy does nothing other than heating the planet.

It is physically and technically almost impossible to convert a higher per centage than the above mentioned 40 per cent of the energy from fossil fuel into electrical energy.

It is, however, possible to make efficient use of the waste heat - for example, for district heating. However, this solution also has negative consequences, because it usually means that an even smaller proportion of the energy in the fuel is converted into electrical energy.

In most cases, just one third (33 per cent) of the potential energy in fossil fuel than is converted into electrical energy.

But in these applications, a larger proportion of the remaining two-thirds of the waste heat can be used for heating greenhouses, buildings, or processes.

This waste heat can also be stored underground as a seasonal energy supply. Processes making efficient use of part of the waste heat are called CHP (combined heat and power).

Location of the CHP

Most electricity is still generated in large power stations with an efficiency of 40 per cent. Where the power station is built is determined by the supply and/or availability of fuels because the electricity can easily be transported over long distances.

In CHP plants, however, the waste heat cannot be transported over long distances. These power stations must therefore be built close to the waste heat users. For that same reason, CHP plants are often smaller, as there is often a limited local capacity to use large amounts of waste heat.

Waste incineration plants are much smaller, for example, than the larger power stations. But these waste incineration plants are very suitable for CHP. The electricity supplied by these waste incineration plants is supplied to the power grid and the waste heat is used nearby.

The electrical efficiency of a power station is mainly determined by two criteria that influence the efficiency of the turbine:

- The higher the pressure and temperature of the steam before the turbine, the higher the efficiency of the turbine, but also:

- The lower the pressure and temperature of the steam after the turbine, the higher the efficiency of the turbine.

High pressure, high temperature

Condensers are used to achieve the lowest possible pressure and temperature after the turbine. Because the steam boiler and the steam turbine in the first case (high pressure, high temperature steam) are relatively very expensive for smaller systems, its operators of smaller systems opt to work with lower pressures and temperatures.

Also, since very low pressure after the turbine can only be achieved when the condenser is very large and very expensive, the operators of small systems often opt for a higher pressure after the turbine.

Together, this means that the electrical efficiency of a combined heat and power plant is usually significantly lower than that of a large (expensive) power station.

Influence the design

Because in most locations it is no longer allowed to use surface water to condense steam, after it has passed through a turbine, nowadays steam is almost always condensed using air-cooled condensers.

The lower the pressure in this condenser, the higher the electrical efficiency of the system. But also the more cooling air is needed to achieve a lower pressure, the larger the condenser needs to be (and the more expensive it will be).

The steam pressure in air-cooled condensers is usually between 0.2 and 0.06bar absolute at a temperature of between 36°C and 60°C. Some of the heat that is still present in the condensed steam can be used for other purposes.

Waste incineration

For many waste incineration plants also have a water purification installation, in which bacteria play an essential role.

These bacteria survive at temperatures between 30°C and 40°C and break down certain substances in the water. If the temperature of the water becomes too high or too low, the bacteria die. It is important, therefore, to keep the water exactly at the right temperature.

The waste heat in the cooling air of the air-cooled condenser is very often exactly right for these types of applications.

However in many cases the processes would employ the waste heat require a higher temperature than the temperature at which the steam comes out of the condenser. This problem can easily be solved by choosing to set the pressure after the turbine a little higher.

Although the electrical yield of the entire system then would decreases slightly, steam becomes available at a higher temperature, making a lot of waste heat available for other purposes. This may in fact be a profitable choice for the total system.

Residual heat

One of our clients, Attero, had an A-frame condenser that was operating at a pressure of 0.07 bar with a corresponding steam temperature of 39°C. When the temperature of the outside air was higher than 15°C, the pressure in the condenser increased, which resulted in lower efficiency of the turbine, causing less electricity to be produced.

Bronswerk developed and delivered an alternative to this system, which taps off some of the steam between the turbine and the condenser. This steam is used to heat water.

As a result, less steam is supplied to the condenser, so less cooling air is required and the ventilators can run more slowly; this means that less driving power is required for the ventilators.

Because the condenser is supplied with less steam, up to an outside air temperature of 19°C the pressure in the condenser can remains low, so that the maximum amount of electricity can be generated for many more hours per year.

In the past, this water was heated by a separate boiler running on biogas. This boiler can now be taken out of service and the biogas can be sold. All in all, the economic efficiency of the entire system has greatly improved.

By proposing and implementing this type of conversion as a total project, Bronswerk's know-how and skill were shown to great advantage and the client only needed a single supplier. The entire project consisted of:

- Tying into the existing steam pipe such that it caused minimal pressure loss

- Designing and manufacturing a new water-cooled condenser

- Supplying the pipe section from the main steam pipe to the condenser

- Supplying the vacuum unit to extract air in the steam

- Supplying condensate pumps

- And supplying a special booster heat exchanger for proper control of the temperature.

This is a good example of how low-temperature waste heat can be used efficiently.

Other possible applications for the efficient use of low-temperature waste heat include: greenhouses; swimming pools; fish farms; drying food or grass or grain; seasonal soil heat.

District heating

For district heating, heat is required at temperatures of up to around 100°C. The criteria described above show that this use of CHP will lead to much lower electrical efficiency.

Another disadvantage of district heating is that the heat requirement varies enormously in summer and winter and during the day and at night.

Needless to say, no advantage is gained from generating electricity with a low efficiency, while on top of that only a small part of the waste heat can be used efficiently. In most systems, this disadvantage is overcome by creating an extra steam tap in the steam turbine. This is used to take steam from the turbine where the pressure is still high. Although this is a good technical solution, it does make the turbine design very complicated.

For this reason, our client Rive Droite Environment (Veolia Environment, Bordeaux, France) decided not to opt for this concept.Instead, they chose to operate the turbine with a low outlet pressure in the summer allowing the maximum amount of electricity to be supplied.

In the summer, there is no need for district heating. In the winter, the outlet pressure of the turbine is increased, leading to the generation of less electricity, with greater amount of heat.

When this combined heat and power plant is in winter mode, most of the steam is supplied to a shell and tube heat exchanger for the district heating. This heat exchanger now operates as a condenser (surface condenser).

However, heat consumption is not constant in winter, so an air-cooled condenser is used to control the system pressure in a simple manner. This solution was supplied complete with a vacuum system, condensate collector and return pipes.

For more information at www.engineerlive.com/ipe

Jeroen van Ruitenbeek and Johan van der Kamp are with Bronswerk Heat Transfer, Nijkerk, The Netherlands. www.bronswerk.com

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