Power Plant Engineering

280 POWER PLANT ENGINEERING

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 this entire 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.

9.5.2 Cogeneration Technologies

(a) 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 kW power to about 4 mW power, 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 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; a gas
boiler supplies the rest.

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.