High Temperature Heat Pump with Combined Cooling, Heat and Power Plant in Industrial Buildings: An Energy Analysis

2.1 Description of the CCHP+HTHP system

The CCHP+HTHP system is set up by:

• the prime mover that can be an internal combustion engine (ICE), a gas turbine (GT), a phosphoric acid fuel cells generator (PAFC) or a molten carbonate fuel cells generator (MCFC). It converts the primary energy of natural gas in input (F_cog) to electricity (E_cog) that can be directed to cover the user’s electricity demand (E_user) and, by means of a suitable heat recovery system, to useful heat (Q_cog). The latter can be used in part (f) to feed the absorption chiller for cooling purposes (Q_abs,gen) and in part direct to the user ((1-f)$\cdot$Q_cog). A small part of energy is wasted as non-useful heat, Q_wasted;
• the absorption chiller (abs) whose generator is fed by a variable (proportional to f) quota of Q_cog and produces cooling power in the evaporator (Q_abs,ev);
• the high temperature heat pump (HTHP) uses the low-temperature heat available from the condenser/absorber of the absorption chiller (Q_abs,cond) as a heat source in the evaporator to produce hot water at 90℃ at the condenser (Q_HTHP,cond). This thermal energy contributes to cover the user heating demand (Q_user).

A schematic of the proposed system is shown in Figure 1, where the main energy flows are reported. Auxiliary units may be necessary such as an auxiliary boiler and an auxiliary electric chiller for covering the heating (Q_{boiler_to_user}) and cooling (Q_chil,ev) demand, respectively.

Once the nominal (electric) power of the prime mover is set, then the nominal heat recovered is known. Consequently, the nominal cooling capacity of the absorption chiller can be defined to use all the recovered heat. Then the nominal capacity of the HTHP is chosen to exploit all the low temperature heat available from the absorption chiller.

The meaning of parameter f is to provide flexibility in the operation of the system: it defines how much of the heat recovered from the prime mover (Q_cog) feeds the absorption chiller (Q_abs,gen=f$\cdot$Q_cog) and, consequently, the quota directly used to satisfy the heating demand. Limit cases are when f=0 (no heat from the prime mover is used to feed the absorption chiller, so there is no cooling production) and f=1 (the cooling production is maximized as all heat recovered from the prime mover feeds the absorption chiller, and the heating demand is covered only by the HTHP).

The electric, heating and cooling production of the system as a function of f is depicted in Figure 2 in normalized terms (fuel input F_cog equal to 1 MW) with given values of the efficiencies of the main equipment. Four cases are reported varying the electric efficiency of the cogenerator and the COP of the HTHP for two typical values (0.35 and 0.45, 3 and 4, respectively) to consider the availability of different technologies. $\eta$_el,cog=0.35 is typical for a GT or PAFC, while 0.45 is typical for an ICE or MCFC [29]. COP_HTHP=3 is a typical value for newer refrigerant (e.g., R1234ze(E)) while 4 is a typical value for ammonia [30]. From Figure 2 it is apparent that electricity production may also be negative, which means that the energy system does not produce electricity, instead it needs electricity from the grid to feed the HTHP. The limit value of f for which the electricity production becomes negative is (Eq. (1), Eq. (2)):

$E_{cog_{to_{user}}}=\eta_{e l, c o g} \;\, F-\left(1+E E R_{a b s}\right) f \eta_{t h, c o g} \cdot\left(\frac{1}{C O P_{H T H P} \;\, -1}\right) F=0$     (1)

$f=\frac{\eta_{e l, c o g} \;\,\; \cdot \; \, \left(\operatorname{COP}_{H T H P} \;\; -1\right)}{\left(1+E E R_{a b s} \; \right) \; \eta_{t h, c o g}}$     (2)

1a.png

CCHP+HTHP

1b.png

SP (BOILER+CHILLER)

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SP (BOILER+HP/CHILLER)

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COGENERATION (CHP)

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TRIGENERATION (CCHP)

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Figure 1. Schematic of the CCHP+HTHP integrated system and the benchmark systems considered in this study

The thermodynamic performance of the proposed trigeneration system (Figure 2) and all benchmark systems described in Section 2.2 (and shown in Figure 1) are evaluated in terms of the following indexes:

• primary energy ratio of the whole system (PER_tot), defined as the ratio between the sum of the useful energy and the sum of the total non-renewable primary energy consumed by the plant (Eq. (3));
• exergy efficiency of the whole system ($\eta$_ex,tot), calculated as the ratio between the exergy flow of the products and the exergy flow in input (Eq. (4) where reference temperature T₀=20℃, useful thermal energy temperature T_Q=90℃, useful cooling energy temperature T_C=10℃).

In order to equitably compare different system configurations, the primary energy (and exergy) input to which the analysis refers is always the fuel chemical energy (which is equal to its exergy). For this reason, the electricity bought from the grid has an exergy efficiency equal to the global thermoelectric efficiency $\eta$_el,ref, that is, the global efficiency of the electricity from the grid produced by a thermoelectric plant fueled by fossil fuels (non-renewable primary energy). Such a parameter will be considered variable in the following analysis to compare different scenarios:

• $\eta$_el,ref=0.4: refers to the “recent past” situation in Italy (say, the first 2000);
• $\eta$_el,ref=0.5: refers to the actual situation in Italy [31];

As depicted in Figure 2, the variation of $\eta$_el,cog and COP_HTHP does not influence the cooling power (C) produced by the system; instead, they determine the electrical and thermal power: for a given value of f, increasing $\eta$_el,cog increases E with Q quite constant, while increasing COP_HTHP reduces Q produced by the system. It should also be noted that the exergy efficiency of the proposed system strongly increases with the increase of the COP of the HTHP and the electrical efficiency of the prime mover.

$PE{{R}_{tot\text{ }}}=\frac{\left( {{Q}_\,{co{{g}_{t{{o}\,_{user}}}}}}\;\;\,+{{Q}_{HTH{{P}\,_{t{{o}_\,{user}}}}}}\;\;\;\;+{{Q}_\,{boile{{r}_\,{t{{o}_{user}}}}}} \;\; \right)+\left( {{C}_{abs\_to\_user}}  \;\;\;\;\, +{{C}_{chil \_to\_user}} \;\;\;\;\;\; \right )+\left( {{E}_{cog\_to\_user}} \;\;\;\; +{{E}_{from\_grid\_to\_user}} \;\;\;\;\;\;\;\; \right )}{{{F}_{cog}} \;\;+\frac{{{Q}_\,{boile{{r}\,_{t{{o}\,_{user}}}}}}}{{{\eta }\,_{boiler\text{ }}}} \;\;\;\; +\frac{{{E}_{from\_grid\_to\_user}}  \;\;\;\;\;\; +{{E}_{from\_grid\_to\_HTHP}} \;\;\;\;\;\;\;\;\; +{{E}_{from\_grid\_to\_chil}} \;\;\;\;\;\;\;\; -{{E}_{cog\_to\_grid}}}{{{\eta }\,_{el,ref\text{ }}}}}$        (3)

${{\eta }_{ex,tot}}=\frac{\left( {{Q}_\,{co{{g}_\,{t{{o}_\,{user}}}}}}+{{Q}_{HTH{{P}\,_{t{{o}\,_{user}}}}}}\;\;\;\;+{{Q}_\,{boile{{r}_\,{t{{o}_{user}}}}}} \;\;\right)\cdot \left( 1-\frac{{{T}_{0}}}{{{T}_{Q}}} \right)+\left( {{C}_{abs\_to\_user}}\;\;\;\;\;+{{C}_{chil\_to\_user}} \;\;\;\;\; \right) \cdot \left( 1-\frac{{{T}_{0}}}{{{T}_{C}}} \right)+\left( {{E}_{cog\_to\_user}} \;\;\;\;\;\;+{{E}_{from\_grid\_to\_user}} \;\;\;\;\;\;\;\; \right )}{{{F}_{cog}} \;+\frac{{{Q}_\,{boile{{r}_\,{t{{o}_\,{user}}}}}}}{{{\eta }_\,{boiler\text{ }}}} \;\;\;+\frac{{{E}_{from\_grid\_to\_user}}\;\;\;\;\;\;\;\;+{{E}_{from\_grid\_to\_HTHP}}\;\;\;\;\;\;\;\;\;+{{E}_{from\_grid\_to\_chil}}\;\;\;\;\;\;\;-{{E}_{cog\_to\_grid}}}{{{\eta }\,_{el,ref\text{ }}}}}$        (4)

2a.png

(a)

2b.png

(b)

2c.png

(c)

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(d)

Figure 2. Power output, energy and exergy performance of the CCHP+HTHP system varying the fraction of cogeneration recovery heat to cooling (f) for 1 MW of NG input ($\eta$_el,ref=0.5)

In the following analysis, an E_user-following operation mode of the CCHP+HTHP system was considered, i.e., the cogenerator is operated to cover the electricity demand from the user (E_user) in all the time step of the simulation, with the electricity from the grid (E_{from_grid_to_user}) covering the possible deficit. In this case, the heat from the cogenerator facing the heating demand (Q_{cog_to_user}=(1-f)$\cdot$Q_cog) and the absorption chiller (Q_abs,gen=f$\cdot$Q_cog) can be not enough, and an integration boiler and electric chiller may be necessary, respectively.

The Q_user-following operation mode was not considered to be interesting: in this case, the cogenerator would be operated to cover the heating demand from the user (Q_user) in all the time steps of the simulation, with the thermal energy produced by the HTHP (Q_{HTHP_to_user}) and the integration boiler (Q_{boiler_to_user}) covering the possible deficit. In this case, no thermal energy would be available to feed the absorption chiller, thus no trigeneration would be realized.

2.2 Description of the benchmark systems

The energy performance of the combined trigeneration high temperature heat pump plant was compared to other benchmark systems for energy production to evaluate the conditions in which the proposed system performed better than traditional ones:

• separate production system with boiler ($\eta$_boiler=0.85) + electric water/water chiller (EER_chil=3 supposed to be constant) (SP Boiler+Chiller);
• separate production system with boiler ($\eta$_boiler=0.85) + electric water/water chiller (EER_chil=3 supposed to be constant) with recovering of the heat of condensation of the chiller to pre-heat the hot water, so decreasing the energy produced by the boiler;
• cogeneration (CHP): The same prime mover of the CCHP+HTHP system was supposed to be operated, with an electric water/water chiller (EER_chil=3 supposed to be constant) to cover the cooling load (C_user) and an integration boiler ($\eta$_boiler=0.85) to integrate the heating load (Q_user);
• trigeneration system (CCHP): the same as the CCHP+HTHP system without the HTHP.

The benchmark systems are depicted in Figure 1.

Table 1 reports the electrical and thermal efficiencies of the four prime movers in partial operation (values at capacity ratio = 1 are the nominal values) [32]. Note that a minimum capacity ratio of 0.3 was considered for all the prime movers.

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Figure 3. Electric (E_user), thermal (Q_user) and cooling (C_user) hourly loads for the annual simulation

In Figure 3 the electric, thermal, and cooling hourly loads for the annual simulation are reported. They approximate the data of the industrial user based on study [28]. The nominal power of the equipment for the simulation are: E_cog=2 MW, C_abs=1 MW, C_chil=5.2 MW, Q_HTHP=3.5 MW, Q_boiler=6 MW.

In this paper, a high temperature heat pump using ammonia as working fluid was integrated within a trigeneration system to produce hot water at 90℃ for industrial uses.

The steady-state simulation was developed both at fixed conditions and on an annual basis. Energy and exergy analysis revealed that the CCHP+HTHP system performs better than the benchmark systems only if the prime mover has a fair good electrical efficiency as in this case it is advantageous to use the electricity produced in cogeneration to feed the HTHP. The quota of heat recovered by the cogenerator to feed the absorption chiller must be optimized to have the best energy performance of the integrated system. The main result is that the higher the electrical efficiency of the cogenerator and the COP of the high-temperature heat pump, the greater is the optimum value of f for the exergy performance of the system to be higher than conventional alternatives.

The integration of the proposed trigeneration system into an existing separate production plant of an industrial user was investigated. It was found that 3.5%, 5.8%, 14.1%, and 22.2% savings were provided compared to traditional separate production (Boiler+HP/Chiller), cogeneration, trigeneration, and separate production (Boiler+Chiller) systems, respectively.

Based on these first results, as a further development of this work a dynamic simulation model of the system will be developed, with the scope of optimizing the fraction of heat to feed the absorption chiller (f) for each time step of the simulation, and optimizing the size of the main equipment. An economic analysis will be useful to assess the economic viability of the proposed energy system and its profitability compared to conventional technologies.