1.1.4.Distributed energy resources
In the last decades a novel technical concept in energy supply emerged, the distributed/decentralized energy resources (DER). DER is defined as an electricity-generation system located in or near user facilities, which provides electrical and thermal energy simultaneously to meet local users in top-priority [15]. they can be divided into two major sections [15]. The first section includes high-efficiency cogeneration or combined cooling and heating (CCHP) systems in industry and buildings, using prime mover technologies as reciprocating engines, gas turbines, micro-turbines, steam turbines, Stirling engines and fuel cells. The second major area of DER is on-site renewable energy systems with energy recycling technologies, including photovoltaic and biomass systems, on-site wind and water turbine generators, plus systems powered by gas pressure reduction, exhaust heat from industrial processes, and other low energy content combustibles from various processes. The combined cooling, heating and power (CCHP) systems are derived from the cogeneration (combined heating and power-CHP) systems. In a CCHP system the thermal or electrical/mechanical energy is further utilized to provide space or process cooling. The CCHP systems are known also as trigeneration systems and as building cooling heating and power (BCHP) systems. One can say that a cogeneration system is a CCHP system without any thermally activated equipment for generating cooling power. Thermally activated equipment is the equipment that uses waste heat instead of electricity to provide air conditioning and/or dehumidification loads such as absorption chiller, adsorption chiller and desiccant dehumidifiers.
The CCHP systems are classified into two categories [15]:
traditional large-scale CCHP systems (predominantly CHP systems without cooling options) in centralized power plants or large industries;
relatively small capacity distributed CCHP units with advanced prime mover and thermally activated equipment to meet multiple energy demands in commercial, institutional, residential and small industrial sectors.
The distributed CCHP systems are classified in accordance with their capacity as follows:
micro systems (capacity under 20 kW);
mini systems (capacity under 500kW);
small scale systems (capacity under 1MW);
medium scale systems (capacity from 1 to 10MW);
large-scale systems (capacity above 10MW).
A typical CCHP system comprises the prime mover, electricity generator, heat recovery system and thermally activated equipment. The diagram of a Stirling engine cogeneration system is shown in Fig. 21. The waste heat from the engine is used to heat up the domestic water, to generate heating power during the winter and to drive the adsorption chiller (for cooling power) during the summer.
The main advantages of distributed CCHP systems are: high fuel energy utilization; low emission; increased reliability of the energy supply network (Table II).
The prime mover selected to meet diverse demands and limitations can be steam turbines, reciprocating internal combustion engines, combustion turbines, microturbines, Stirling engines and fuel cells. The thermally activated systems include absorption chillers, adsorption chillers and desiccant dehumidifiers.
A comparative presentation of main cogeneration systems is shown in Fig. 22. This diagram is useful in choosing the cogeneration system for a given application (electrical power demand).
Figure 21. Schematic diagram of a micro combined cooling, heating and power system.
Figure 22. Distribution of main cogeneration systems.