Repsol YPF

Electricity and heat generation: Cogeneration

The term 'cogeneration' is used to define processes in which electrical energy and heat energy are produced simultaneously, using a fuel which is either gas-oil or gas.

The electrical energy is produced by the engine when the alternator attached to it is activated, whereas heat generation is achieved by making use of the residual engine heat, such as the heat extracted from the engine's cooling systems or exhaust.

The requirements needed to be met in order for a cogeneration plant to be operationally profitable are:

• That the heat and electricity demands are simultaneous, or that the former can be accumulated.
• That both demands are proportional to the production of the heat engine (whether engine or turbine).
• That the surplus is minimal or does not have negative repercussions on the profitability.  

The electrical energy generated can be used to cover internal consumption or exported to the electricity network. In the former case, costs can also be reduced with regard to the electrical power installed. On the one hand, the plant's profitability depends upon the investment necessary -and in this respect the investment necessary for outputs of under 500kW, with all the equipment that this entails, exchangers, towers, recuperators, etc., is very high in relation to the €/kW installed- and on the other hand, the prices of the electricity and of the fuel used in the engine are two fundamental parameters in establishing the profitability of an investment of this type.

Cogeneration plants are sized, in general, according to the demand for heat energy, and therefore it is necessary to analyse the annual development of this heat demand and determine the relevant operating parameters. The following are the general rules:

1. The heat output of the cogeneration plant must cover approximately 30-50% of the maximum heat demand. In this case, as demonstrated by experience, the plant's modules cover approximately 50-70% of the annual heat energy requirement, with the rest being provided by additional boilers which cover the heat consumption peaks.
2. Each cogeneration module must reach a minimum service time of 4,000 hours per year.

A cogeneration system is made up of an engine or gas turbine connected to an electrical alternator, the performance of which is around 96%, compared with 35% for thermoelectric plants.

More specifically, in the case of cogeneration using propane gas, the cogeneration system is made up of an internal combustion engine, an electrical energy generator, a recovery system for the heat energy coming from the exhaust gases or the hot water in the cooling system, etc., and an evacuation system for residual smoke.

These systems have quite a wide power range and adapt well to variable loads through the addition of various modules, of which, at each moment in time, only those that are necessary are in operation. Furthermore, there are gas cogeneration systems that can replace emergency generating sets and, in general, their installation does not require a greater investment than for traditional complementary supplies. In fact, they actually provide a considerable saving compared to the latter. Therefore, for the service sector, it is generally worthwhile investing in these systems, due to their variable demand characteristics and range of power outputs, especially in cases in which either the legislation or the company's own interest requires emergency generating sets to be installed. In the latter case, there is a clear economic interest involved, as well as the fact that the installation of a cogeneration system increases the reliability of the emergency set.

The overall efficiency of these systems can reach averages of between 75 and 90%, depending on the quality and quantity of the heat recovery: normally water at 80º or 90º. This hot water is obtained by harnessing the heat from the engine coolant, from the lubricating oil and lastly, from the exhaust gases that can be evacuated into the atmosphere at 90º without condensation being produced.