TRIGENERATION

The excess heat is produced as a by-product of electricity generation, but chilled water is required, absorption chillers are the answer. Their apparently low efficiency is compensated for by the fact that the input energy is low quality (and therefore cheaper) heat, rather than the high quality electricity required by a turbo-chiller. The typical maintenance requirement is approximately 25 hours per annum. A typical life span is 20 years, as opposed to 15 years for turbo-chillers and 10 years for chillers employing a reciprocating compressor.


HISTORICAL DEVELOPMENT.
At the beginning of the twentieth century most industrial consumers of electricity generated their own power. At the time, the electric utility service was unreliable, expensive, and not widely available. On-site generation was usually a better alternative. In the early 1900’s over half of all the electricity consumed in the United States was self-generated. Of this, half was from cogeneration systems (25% of total). A number of factors then combined so as to cause cogeneration to decline greatly in significance. It reached its nadir in 1980, with only 3% of electrical power in the U.S.A. being produced by cogeneration. The factors resulting in this state of affairs were as follows:

 

 

 

 


RESURGENCE IN INTEREST.
In recent years, a number of developments have resulted in renewed interest in the concept of cogeneration and trigeneration. These developments are:

           

 

 

 

 


SUPPLY INDUSTRY LIBERALIZATION.
Traditionally, electric utilities have discouraged the use of customers generating their own electricity by:

 


In addition, general industry has been reluctant to engage in power generation because this is not its main business and they are afraid of being regulated. As a result of political pressure, utilities are being forced to change their attitude, with a consequent dramatic reduction in the price of electricity.

WHERE AND WHEN TO USE
Consideration should be given whenever there is a simultaneous requirement for electrical and thermal energy. System capacities have been installed down to a minimum level of 15 kw (e). Electricity generated should be consumed on the operator’s premises, since the cost of grid electrical power substituted is usually considerably higher than the rate at which it can be sold to the power company. Optimal performance will be achieved at a thermal to electrical energy consumption ratio that corresponds with the respective heat-to-power ratio of the prime mover selected.

Maximum system efficiency is achieved when a unit is operating at full load for as long as possible. This requires a continuous heat demand throughout the year. Consequently, an important factor when sizing CHP systems is the level of heat demand. As a rule of thumb, to arrive at a payback period of 3 to 4 years, a CHP system has to run for about 4.500-6,000 full-load hours per annum. Systems that are oversized will either have to run for long periods at part load or operate for a reduced number of hours per year. Thermal efficiency is maximized when units are controlled to follow the heat load. This is the philosophy most popular with industrial and commercial users. Obviously, overall efficiency will be penalized in situations where dump radiators are used to allow maximum electricity generation at times of low heat demand. Units should be sized at a capacity that will ensure a high level of utilization, with additional heat demand being met by conventional boilers.

- Larger and more efficient utility power stations that lowered the cost of power.
- Such power stations tended to be located remotely from the heat loads, i.e. near coalfields or in coastal areas.
- Extension of the public supply networks, together with an improvement in service and reliability.
- Utilities offered very low prices for excess power sold to them by an industry.
- Utilities charged high prices for standby or supplemental power required by the self-generator.
- Electricity generation became a regulated business and on-site generators could be regulated as a utility if they   sold any excess power.
- With energy prices cheap, the spending of capital by industry to save fuel was difficult to justify economically.
- Larger and more efficient utility power stations that lowered the cost of power.
- A large escalation in energy prices, with the need to reduce consumption.
- Increased public and political awareness of the environment. This has resulted in regulations aimed at reducing   greenhouse gas emissions. It has also resulted in the current international debate, with the same objective.
- Liberalization of the electricity supply industry. This has enabled self-generators to sell their excess power at   economic rates, without being regulated as an electric utility. It has also resulted in fair treatment by local   utilities with regard to interconnection and back-up power.
- Technological limits on the maximum size of power plants.
- The development of new technologies that are more efficient or lower in capital cost, e.g. gas turbine combined   cycle, first installed in the late 1960s.
- Problems experienced by high-tech industries with the power quality available from public supply networks.
- The recent development of low cost, prepackaged cogeneration systems in small sizes is opening up the   market for relatively small energy users.
- Refusing to buy back electricity.
- By offering a very low buy back price.
- Requiring a high demand charge for the back up power provided.
This provides electricity, heat and chilled water from a single system. If electricity and heat alone are generated, the concept is known as cogeneration. A general term for such systems is combined heat and power (CHP). By utilising the waste heat, such systems make more efficient use of fuel than the usual generation methods. The conventional method of generating electricity using fossil fuel has an efficiency of approximately 35%. By comparison CHP has a typical overall efficiency of 85%. This results in primary fuel savings of around 50%. This higher efficiency results not only in direct fuel savings but also in lower consumption of fossil fuels and hence lower emission of carbon dioxide (CO2).