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Case History: Cogeneration and Trigeneration
A thermal power plant - gas, coal or nuclear - uses heat in order to generate electricity.
Because the power plant is usually located some distance from its consumers,
much of this heat goes to waste, either through cooling towers or,
in the case of many nuclear power stations, into the sea.
The idea of supplying this waste heat as “district heating” is not new.
A hundred thousand buildings in New York receive heat from the local Con Edison power plants.
District heating is also common in Eastern Europe.
The principle of supplying both heat and power from a single plant
is called “Cogeneration” or “Combined Heat and Power”.
This has the potential to increase the efficiency of the power station from 40-50%
to 60-80%. That means that much less of the energy value of the fuel goes to waste.
However, there are going to be times when heat is not required
- particularly in hotter countries.
One answer is “trigeneration” or “polygeneration”. This third output is district cooling.
The power station uses heat to power absorption chillers,
as opposed to the compressor refrigeration process used in most air conditioning units and refrigerators.
Further ecological benefits can be achieved by using renewable fuel sources
as opposed to fossil fuel. The renewable fuel sources for a thermal power station
might be biogas, biomass, solar or geothermal energy.
Case History
At the centre of the 84 hectare development is the Energy Park
a tri-generation complex which supplies nearly two thirds of the energy requirements
of the apartments on the site.
Each apartment is connected to a grid supplying hot water for underfloor heating
in the winter and chilled water for cooling in the summer.
All the energy sources are renewable. Perhaps the most innovative of these is geothermal energy.
We are in the power station where almost all the energy is concentrated and distributed.
Here, on my left, is the terminal of the geothermal probes.
These are coming from the geothermal field, where we have over 200 probes
gathering energy from a depth of over 150 metres.
They’re coming in here through those holes we can see on the outer wall.
Then, through heat exchangers, these probes deliver the heat into this machinery.
That allows us to produce a high proportion of energy from renewable sources.
Geothermal, quite simply, is useful because we can recover heat from it. It’s a benefit.
Let’s give an example. An elevator takes people from the ground floor up to the sixth floor.
With a traditional system, you get in on the ground floor, you start the lift motor
and it works until you get to the sixth floor.
With geothermal energy, it’s like starting the motor from the 1st or 2nd floor.
We use less electricity. We’re using the heat energy we get from the gound.
The ground is like a reservoir. In the winter we can recover heat at 18-20 degrees.
The heat pump raises this from 20 to 30-35 degrees for distribution.
That’s a jump of 15 degrees.
If we don’t recover this heat from the ground, we’ll have to lift the temperature
from zero to 35 degrees, which is more challenging and we’ll have to pay for this.
That’s the great advantage of geothermal energy. It uses heat that otherwise
would go unused, wasted.
To generate electricity, they use diesel generators, running on vegetable oil.
The waste heat from the diesel engines is also recovered.
These engines run on vegetable oil. Jatropha oil.
In effect, we are able to produce electricity and thermal energy at the same time.
That’s something that’s very advantageous. As we generate electricity, we are producing
enough excess heat to allow us to supply domestic hot water to the apartments.
Otherwise all this energy would be thrown away, dissipated into the atmosphere.
So we would actually be adding to air pollution, rather than making use of it
to produce domestic hot water.
We get heat from the coolant that this engine needs while it’s running,
and we’ve also installed equipment that recovers heat from the exhaust gases.
The exhaust gases are coming out of this cogenerator at 400, 500, 600 degrees.
We pass them through a heat exchanger module. Cold water passes through
and the gases effectively heat the water. Then cooled gas goes into the atmosphere.
In practical terms, we pass the gases through a catalytic converter
which turns the NX into urea and water. And we filter out particles and pollutants.
For this project, our considerations were part energy, part environmental and part noise.
The whole exhaust system is contained in a silencing chamber, so that we can ensure
the highest environmental and accoustic performance, because there are residential
buildings above here, and we have to ensure the well-being of the people who live in them.
We must ensure that the buildings are not polluted with noise or fumes, but at the
same time we want the residents to benefit from the energy from these systems.
Combined with geothermal and biodiesel energy are photovoltaic panels.
The construction of the apartments themselves is highly energy efficient.
The energy park produces approximately 6.5 thousand megawatt hours per year
from renewable sources, while at the same time reducing the CO2 output of the housing
development by some 70% when compared to traditional energy generation.