2. History

resources are not only limited but also are detrimental to our environment [2]. Among different power consumers, buildings are major energy sink comprising 40% of total U.S. energy consumption [3]. Thus, the increasing demand for sustainable buildings with the constant need of cooling and heating power in buildings calls for improving traditional energy production and optimum use. One method to produce sustainable energy is to adopt the combined cooling, heating, and power (CCHP) technology, which is also known as trigeneration. Today, the CCHP system has proven effective in ensuring energy savings, as well as reducing the emission of pollutants [4]. This technology is a more advanced form of the combined heating and power (CHP) system and is becoming widely accepted with consumers. While a CHP system involves the simultaneous production of two types of energy such as electricity and heat, usually in the form of either hot water or steam, from one primary fuel, such as natural gas; the CCHP system, as the name implies, produces three forms of energy: electricity, heat, and chilled water [5]. Chilled water is achieved by incorporating an absorption chiller into a cogeneration system. Absorption chillers use the waste heat from a CCHP system to create chilled water to cool buildings. Introducing an absorption chiller into a CHP system allows a site (e.g., buildings) to increase its operational hours through the increased use of heat, which ultimately reduces energy costs [6]. Because of its abilities to save energy, reduce emissions, and provide economic benefits,

Burning fuels such as natural gas or coal results in significant amounts of heat energy and waste materials. Generally, a mechanical apparatus converts the heat energy into electrical energy [7]. However, a significant portion of heat energy is wasted and discharged into the environment [8], and such unused heat energy has significant potential that a CCHP system exploits. First, CCHP accomplishes cooling that is used to provide air conditioning, as the heat produced during electricity generation can be used to drive absorption chillers. Second, the CCHP makes maximum use of the waste heat from the prime movers to supply heat to the buildings and provide hot water for industrial processes. In this way, a CCHP system maximizes heat energy use in buildings and increases the prime mover efficiency. In the literature, it was reported that CCHP systems could yield efficiencies more than twice that of average power plant efficiency [9–11]. On the contrary, this percentage is not always constant. The electrical load may remain almost constant throughout the year and thus can maintain a certain level of fuel consumption. However, the demand for cooling and heating varies throughout the year. The demand for cooling is higher during summer and that for heating is higher during winter. However, during spring and fall, the need for both cooling and heating may decrease significantly, and in such cases, the efficiency of the CCHP system may decrease. However, this technology allows greater operational flexibility at sites (e.g., buildings) that demand energy in the form of heating as well as cooling [12]. That specific benefit is attractive in tropical countries where buildings need to be air-conditioned in all seasons as well as to industries that require process heating and cooling over the year. Finally, a CCHP system generates power in a way similar to that of conventional systems and can be utilized as a backup power system. This also reduces fuel and energy costs and CO2 production compared to electricity produced from coal. All of these advantages have made the CCHP systems an economically viable alternative to produce power as well as to condition the building environment [13]. This chapter describes the history of CCHP, provides basic CCHP configuration,

the CCHP system has attracted much attention worldwide.

42 Energy Systems and Environment

Since the beginning of the electric age, power plants produced far more heat than electricity. In 1882, Thomas Edison used cogeneration of both steam and electricity in the world's first commercial power plant in New York [14]. Then, at the beginning of the twentieth century, steam became the principal source of mechanical power [15]. At the same time, energy became more controllable and many small power houses that produced steam to customers for space heating or industrial use realized that they might also produce electricity as well [16]. Because steam cannot be transported far without a significant loss of heat, cogeneration was dependent on a district energy strategy for small community plants. After World War II, there was significant growth in centralized power plants that could deliver electricity over a wide region [17]. During 1940–1970, the concept of a centralized electric utility that could deliver power to the surrounding area was developed, and as a result, steam no longer was a viable commodity. During that time, large utility companies became both reliable and comparatively inexpensive sources of electricity. That situation caused small power houses to stop using the CHP system and instead, they bought their electricity from the large utility companies. Further, as central utilities became more reliable and less costly, CHP remained economical only in industries that required large amounts of steam.

During the late 1960s and early 1970s, interest in CHP began to revive, and by the late 1970s, the need to conserve energy resources became clear [18]. During this time, legislation was passed in the United States to promote cogeneration because of its efficiency. Specifically, the Public Utilities Regulatory Policies Act (PURPA) of 1978 encouraged this technology by allowing CHP producers to connect to the utility network and to purchase as well as sell electricity. In times of shortfall, PURPA allowed CHP producers to buy electricity from utility companies at fair prices and also allowed them to sell their electricity based on the cost the utility would have paid to produce that power [19]. These conditions encouraged a rapid increase in CHP capacity in the United States. However, at that time, there was little government support for CHP in Europe because the cogeneration was not seen as a new technology 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 costs, and fewer related emissions.

and low pressure) discharged from the prime mover to drive the adsorption chillers and thus

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

http://dx.doi.org/10.5772/intechopen.77000

45

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

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].

reduce the primary energy consumption overall.

4. Prime movers

combustion.
