5. Performance parameters of CCHP

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 to the reference quantities.

#### 5.1. Economic analysis

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

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 ulti-

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 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

chilling machine. The other mechanisms are similar to those in the ICE system.

mately, the mechanical energy is transformed into electricity.

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

the atmosphere.

46 Energy Systems and Environment

low temperatures [28].

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

$$A\mathbf{O}\mathbf{C}\_{PM} = \sum\_{i=1}^{8760} F\_{mi}\mathbf{C}\_{NG} + E\_{grid\_i}\mathbf{C}\_{elec} + P\_{PM\_i}\mathbf{C}\_{om} \tag{1}$$

$$AOC\_{ref} = \sum\_{i=1}^{8760} F\_{mref\_i} \mathbf{C\_{NG}} + E\_{grid\_{ref\_i}} \mathbf{C\_{elec}} \tag{2}$$

$$AS = AOC\_{ref} - AOC\_{PM} \tag{3}$$

PECs <sup>¼</sup> <sup>X</sup> 8760

5.3. Emission characteristics

i¼1

respectively. Values for this study are given in Table 1.

relative to the reference system, are represented by [33]:

when the CCHP system is operated and can be calculated by

Fmref <sup>i</sup>

PFNG <sup>þ</sup> Egridref <sup>i</sup>

Ems,g <sup>¼</sup> <sup>X</sup> 8760

i¼1

Fmref <sup>i</sup>

PFelec � � � FmiPFNG <sup>þ</sup> Egridi

where PFelec and PFNG are the primary energy conversion factors for electricity and natural gas,

The equations for the reduction in emissions for all three gases considered in this study,

Here, g in the subscripts represents the gas for which the savings are being calculated, i.e., represents the emission savings for carbon dioxide (g = CD), nitrogen oxides (g = NX), and methane (g=M) are the emissions from the reference case and are the emissions obtained

where, EFNG,g and EFelec,g are the emission factors for the respective gases from natural gas and electric sources as shown in Table 1. Emission conversion factors tabulated in Table 1 can be used to determine the overall emissions of CO2, NOx, and CH4. The installation location of the PM in the CCHP system and fuel types required for electricity influence the emission

Variable Symbol Value Unit Electric cost Celec 0:0757 \$=kWh Natural gas cost CNG 0:0125 \$=kWh Electric CO2 emission EFelec,CD 0:682 kg=kWh Natural gas CO2 emission EFNG,CD 0:181 kg=kWh Electric NOx emission EFelec,NX <sup>1</sup>:<sup>12</sup> � <sup>10</sup>�<sup>5</sup> kg=kWh Natural gas NOx emission EFNG,NX <sup>8</sup>:<sup>54</sup> � <sup>10</sup>�<sup>7</sup> kg=kWh Electric CH4 emission EFelec,M <sup>8</sup>:<sup>26</sup> � <sup>10</sup>�<sup>6</sup> kg=kWh Natural gas CH4 emission EFNG,M <sup>1</sup>:<sup>17</sup> � <sup>10</sup>�<sup>8</sup> kg=kWh Electric PEC factor PFelec 3:5 — Natural gas PEC factor PFNG 1:09 —

Table 1. Cost of fuel and electricity, gas emissions as well as PEC factors for Minneapolis, MN [32].

PFNG <sup>þ</sup> Egridref <sup>i</sup>

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

Emref <sup>i</sup> � EmCCHPi Emref <sup>i</sup>

PFelec � �

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

(9)

49

(10)

PFelec

EmCCHP ¼ FmEFNG, <sup>g</sup> þ EgridEFelec, <sup>g</sup> (11)

Emref ¼ Fmref EFNG,g þ Egridref EFelec,g (12)

As shown in Eq. (4), the calculation of the simple payback period (SPP) depends on the AS calculation [32].

$$SPP = \frac{IC}{AS} \tag{4}$$

where, IC is the initial cost. A discounted cash flow method, such as internal rate of return (IRR), is also used to evaluate these CCHP systems. CCHP is attractive for building operations when IRR is greater than the minimum attractive rate of return (MARR). IRR can be calculated from the Eq. (5).

$$I\mathcal{C} = \text{AS} \left[ \frac{(1 + IRR)^{L\_{PM}} - 1}{IRR(1 + IRR)^{L\_{PM}}} \right] \tag{5}$$

where, LPM is the lifetime of the PM [29]. Another discounted cash flow method is the net present value (NPV) for CCHP systems. NPV can be calculated as shown in Eq. (6):

$$NPV = \sum\_{n=0}^{N} \frac{AS}{(1+i)^n} - IC \tag{6}$$

where, i is the discount rate, n is the time of cash flow (period), and N is the total number of periods. A third analysis that uses discounted cash flow is the equivalent uniform annual savings. First, the equivalent uniform annual cost is determined according to

$$ELAC = IC \frac{\xi (1 + \xi)^{L\_{\rm PM}}}{(1 + \xi)^{L\_{\rm PM}} - 1} \tag{7}$$

where, ξ is the interest rate, chosen as a representative value for bank offered rates. Equivalent uniform annual saving can then be calculated from

$$ELAS = ELAC - AS \tag{8}$$

#### 5.2. Energy consumption

Savings in primary energy consumption can be calculated by

CCHP System Performance Based on Economic Analysis, Energy Conservation, and Emission Analysis http://dx.doi.org/10.5772/intechopen.77000 49

$$\text{PEC}\_{s} = \sum\_{i=1}^{8760} \frac{\left(F\_{mref\_i} PF\_{NG} + E\_{grid\_{ref\_i}} PF\_{elec}\right) - \left(F\_{mI} PF\_{NG} + E\_{grid\_i} PF\_{elec}\right)}{F\_{mref\_i} PF\_{NG} + E\_{grid\_{ref\_i}} PF\_{elec}} \tag{9}$$

where PFelec and PFNG are the primary energy conversion factors for electricity and natural gas, respectively. Values for this study are given in Table 1.

#### 5.3. Emission characteristics

AOCPM <sup>¼</sup> <sup>X</sup>

calculation [32].

48 Energy Systems and Environment

from the Eq. (5).

8760

i¼1

8760

i¼1

AOCref <sup>¼</sup> <sup>X</sup>

FmiCNG þ Egridi

Fmref <sup>i</sup>

As shown in Eq. (4), the calculation of the simple payback period (SPP) depends on the AS

SPP <sup>¼</sup> IC

where, IC is the initial cost. A discounted cash flow method, such as internal rate of return (IRR), is also used to evaluate these CCHP systems. CCHP is attractive for building operations when IRR is greater than the minimum attractive rate of return (MARR). IRR can be calculated

IC <sup>¼</sup> AS ð Þ <sup>1</sup> <sup>þ</sup> IRR LPM � <sup>1</sup>

where, LPM is the lifetime of the PM [29]. Another discounted cash flow method is the net

present value (NPV) for CCHP systems. NPV can be calculated as shown in Eq. (6):

NPV <sup>¼</sup> <sup>X</sup> N

savings. First, the equivalent uniform annual cost is determined according to

uniform annual saving can then be calculated from

Savings in primary energy consumption can be calculated by

5.2. Energy consumption

n¼0

where, i is the discount rate, n is the time of cash flow (period), and N is the total number of periods. A third analysis that uses discounted cash flow is the equivalent uniform annual

EUAC <sup>¼</sup> IC <sup>ξ</sup>ð Þ <sup>1</sup> <sup>þ</sup> <sup>ξ</sup> LPM

where, ξ is the interest rate, chosen as a representative value for bank offered rates. Equivalent

IRRð Þ <sup>1</sup> <sup>þ</sup> IRR LPM " #

AS

Celec þ PPMi

AS ¼ AOCref � AOCPM (3)

AS (4)

ð Þ <sup>1</sup> <sup>þ</sup> <sup>i</sup> <sup>n</sup> � IC (6)

ð Þ <sup>1</sup> <sup>þ</sup> <sup>ξ</sup> LPM � <sup>1</sup> (7)

EUAS ¼ EUAC � AS (8)

CNG <sup>þ</sup> Egridref <sup>i</sup>

Com (1)

(5)

Celec (2)

The equations for the reduction in emissions for all three gases considered in this study, relative to the reference system, are represented by [33]:

$$Em\_{s,g} = \sum\_{i=1}^{8760} \frac{Em\_{ref\_i} - Em\_{CCHP\_i}}{Em\_{ref\_i}} \tag{10}$$

Here, g in the subscripts represents the gas for which the savings are being calculated, i.e., represents the emission savings for carbon dioxide (g = CD), nitrogen oxides (g = NX), and methane (g=M) are the emissions from the reference case and are the emissions obtained when the CCHP system is operated and can be calculated by

$$Em\_{\rm CCHP} = F\_m EF\_{\rm NG,g} + E\_{\rm grid} EF\_{\rm elec,g} \tag{11}$$

$$Em\_{ref} = F\_{mref} EF\_{NG,g} + E\_{grid\_{ref}} EF\_{elec,g} \tag{12}$$

where, EFNG,g and EFelec,g are the emission factors for the respective gases from natural gas and electric sources as shown in Table 1. Emission conversion factors tabulated in Table 1 can be used to determine the overall emissions of CO2, NOx, and CH4. The installation location of the PM in the CCHP system and fuel types required for electricity influence the emission


Table 1. Cost of fuel and electricity, gas emissions as well as PEC factors for Minneapolis, MN [32].

conversion factors. Emission is also observed in the reference system because of the grid electricity generation produced originally in the power plant. Emissions of the reference system are also due to the local boiler. Three factors dominate the emissions caused by CCHP: (i) electricity produced by the CCHP systems, (ii) electricity generation process of the power plant, and (iii) heat produced by the boiler.

by government entities. The economic benefits of the CCHP system can also be significantly affected by local climate conditions since it changes the building heating and cooling demand. Generally, the parameters used to determine economic benefits are the simple payback period (SPP), annual savings (AS), internal rate of return (IRR), and equivalent uniform annual savings (EUAS). Previous research has shown that the CCHP system is able to satisfy the energy demands of a building when it is integrated with the electric grid to achieve positive values of EUAS, IRR, and AS [32]. Figure 5 shows the economic benefits for the three different prime movers in a case study conducted in Minneapolis, MN. The reciprocating internal combustion engine (ICE) demonstrated the greatest economic benefits overall across all building types. It also resulted in the best IRR values among the three prime movers. Moreover, the reciprocating ICE provided the maximum savings based on the EUAS values calculated. Based on the study, a fuel cell was the least economically advantageous and resulted in negative EUAS values for all building types. The reason for the net loss is attributable to the high capital cost of the fuel cell. However, the selection of a new prime mover for the CCHP generally depends on the analysis of economic parameters, as well as project details. Further, budget restrictions, credits for energy saving, and capital incentives need to be considered when

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

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51

The CCHP system is an effective way to save energy over customary system with separate cooling and heating systems as it uses prime mover exhaust to heat and cool the building. This provides an alternative for the world to meet and solve energy-related problems, such as energy shortages and supply security, emission control, etc. Comprehensive analysis is often warranted to decide on appropriate prime mover for a CCHP system, which relies on the tradeoffs between energy savings, environmental impacts, and economics benefit. CCHP system's energy performance is greatly depends on the site weather zone, it works with maximum efficiency where heating, cooling, and electricity demands are mostly uniform through most or all of the year. However, energy savings will be significantly high if the installation site has higher heating demand, as it is more efficient to utilize the low quality thermal energy from PM exhaust to heat the facility rather use that

Generally, the energy conservation parameter for the study is the primary energy consumption (PEC) [32]. Another parameter, referred to as site energy consumption (SEC) always increases when the CCHP is used [33]. In contrast, the PEC is a better indicator of energy feasibility because of its potential to decrease when the CCHP is operational [33]. Figure 6 shows the PEC results of the energy analysis in the case study conducted in Minneapolis, MN, where the reciprocating ICE and fuel cell showed almost similar energy (PEC) savings. All types of buildings experienced reductions in PECs when a CCHP system was adopted. When only the primary energy savings are considered in the absence of an economic analysis, all three prime

selecting the prime mover.

7. Energy conservation

energy to cool the building.

movers are good options for the three building types.
