4. Prime movers

and therefore was not covered under the European Community's energy program. However, some individual European countries, like Denmark and Italy, adopted separate energy policies that allowed them to incorporate CHP facilities in their future energy projects. At present, the EU generates 11% of its electricity using cogeneration [20]. Because of the price increment of energy types on the market and the need for heating and cooling energy in modern buildings, considerable research has been conducted to improve the CHP system [21]. The historical basis and success of CHP then led to further steps to expand the efficiencies of CHP to CCHP, as each new increase in energy recovered will result in higher efficiencies, lower fuel/energy

Combined cooling, heating, and power (CCHP) systems consist of a decentralized power generation source where a portion of the heat released as a byproduct of generation eventually gets recovered rather than rejected to the atmosphere. There are four main units of a CCHP system: (a) power generation unit, which is referred to as the plant's prime mover, such as a gas turbine, (b) cooling unit, such as a single-effect absorption chiller, (c) a heating unit, such as

In the typical CCHP system, mechanical power is produced from a thermal generation unit, such as a gas turbine. The mechanical power produced gets utilized to rotate an electrical generator. The generation unit produces waste heat, including exhaust gases and lubrication oil that is recovered to meet the cooling and heating demands of the building or industrial unit. One portion of waste heat is used to meet the heating demand, such as a building's heating load, while the remaining portion is used to meet the cooling demand. Moreover, cooled water from the chiller is used as a working fluid for the heat supply from the condenser and absorber of the chilling machine. CCHP systems provide cooling by using low quality heat (low temperature

costs, and fewer related emissions.

44 Energy Systems and Environment

Figure 1. Schematic of a typical CCHP system.

3. Basic CCHP system design configuration

the boiler, and (d) electrical generator as shown in Figure 1.

The adoption of the CCHP system in buildings is mainly dictated by the main component of the CCHP system, its prime mover. Other components of the CCHP system (e.g., heating unit and cooling unit) do not have significant effects on its adoption in buildings. Several types of prime movers have been utilized for CCHP systems, including internal and external combustion engines, steam, gas, and microturbines, and fuel cells [22]. These different types of prime mover distinguish one CCHP from another. The number of prime movers varies depending on the electricity load demand. Operating with more than one fuel type adds flexibility to the prime mover's operation. However, the fuel type affects the greenhouse gas emission rate. For example, natural gas combustion produces fewer greenhouse gas emissions than do diesel combustion.

An internal combustion engine (ICE) system (Figure 2a) is the most common type of a prime mover. The merit of ICE systems depends on how often CCHP generation is required [23]. In this system, heat can be recovered from exhaust gases and the engine's cooling circuit. Moreover, heat is generated from exhaust gases for the absorption chilling machine. Cold water limits the operating temperature of the engine and uses thermal energy from exhaust gases in

Figure 2. Simplified scheme of a trigeneration plant with (a) internal combustion engine with absorption chilling machine and (b) gas turbine with absorption chilling machine [23].

the heat exchanger to generate hot water or steam. In most cases, it is used to produce cooling energy by electric refrigerators. On the other hand, when the prime mover is a gas turbine (Figure 2b), the turbine generates electricity. In this case, heat generated from exhaust gases can be delivered to the users and a portion of it is used as a driving force for the absorption chilling machine. The other mechanisms are similar to those in the ICE system.

The prime mover of a steam turbine CCHP system is a steam boiler that needs fuel and air input to produce high pressure steam that feeds the steam turbine. When steam expands in the steam turbine, a portion of the thermal steam energy is transformed into mechanical energy. Moreover, the rotor of the electric generator is connected to the same turbine shaft, so ultimately, the mechanical energy is transformed into electricity.

The CCHP system design with microturbines is slightly older and dates back to the twentieth century [21]. Microturbines are small electricity generators that burn gaseous and liquid fuels to create high-speed rotation that turns an electrical generator. These are ideal prime movers for decentralized CCHP systems with small-scale rated power (Figure 3). This system has attracted attention because it has several benefits over other prime movers. The size range for microturbine available and in development is from 30 to 400 kilowatts (kW), while conventional gas turbine sizes range from 500 kW to 350 megawatts (MW) [24]. Moreover, microturbines run at high speeds and, like larger gas turbines, are able to operate on a variety of fuels, including natural gas, sour gases (high sulfur and low Btu content), and liquid fuels, such as gasoline, kerosene, and diesel fuel/distillate heating oil [25]. In resource recovery applications, they burn waste gases that otherwise would be flared or released directly into the atmosphere.

CCHP systems with a fuel cell as a prime mover are promising because of their potential to generate electricity and thermal energy directly. In this system, membrane steam reformers purify hydrogen without cooling the reactor's effluent. Before electrooxidation at the fuel cell's anode, only permeated hydrogen needs to be cooled. Both the absorption and compression heat pumps use the fuel cell's waste heat and electricity for heating and cooling applications [29]. Moreover, high-temperature fuel cells combined with an absorption chiller offer the

CCHP System Performance Based on Economic Analysis, Energy Conservation, and Emission Analysis

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In order to determine the best performance parameters and boost the performance for the CCHP system, several equations have been applied. Equations to determine the GHG emissions [e.g., carbon dioxide (CDE), nitrogen oxides (NXE), and methane (ME)] have been set as well. Moreover, methods to calculate the annual cost savings and primary energy consumption (PEC) can also be represented with appropriate equations and are presented in [31]. All of these equations to calculate the performance parameters are presented in this section. The annual cost savings have been reported as dollar amount and the CDE, NXE, ME, and PEC were reported in terms of "relative savings" with respect

Eq. (1) can be used to calculate the total annual operating cost (AOC) of the CCHP system together with the reference system. Parameters CNG and Celec used in Eqs. (1) and (2) are the cost of natural gas and electricity, respectively. The operational (excluding fuel) and maintenance cost per unit of energy produced by the PM is designated as COM. The value represents the energy produced during the ith interval. The annual savings can be calculated by

potential to meet the criterion of virtually zero pollutant emissions [30].

Figure 4. CCHP system design with the Stirling engine as a basic aggregate [21].

5. Performance parameters of CCHP

deducting AOCPM from the AOCref as shown in Eq. (3).

to the reference quantities.

5.1. Economic analysis

The CCHP system that uses the Stirling engine (Figure 4) as a prime mover can be used as energy sources for small commercial and residential buildings. It can operate with a wide variety of fuels, including all fossil fuels, biomass, solar, geothermal, and nuclear energy [26]. The external combustion that controls the combustion process results in low emissions, noise, and waste heat flow [27]. Another major advantage of the Stirling engine is that it can work at low temperatures [28].

Figure 3. CCHP system design with a microturbine as a basic aggregate [21].

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Figure 4. CCHP system design with the Stirling engine as a basic aggregate [21].

CCHP systems with a fuel cell as a prime mover are promising because of their potential to generate electricity and thermal energy directly. In this system, membrane steam reformers purify hydrogen without cooling the reactor's effluent. Before electrooxidation at the fuel cell's anode, only permeated hydrogen needs to be cooled. Both the absorption and compression heat pumps use the fuel cell's waste heat and electricity for heating and cooling applications [29]. Moreover, high-temperature fuel cells combined with an absorption chiller offer the potential to meet the criterion of virtually zero pollutant emissions [30].
