Energy-Exergy Analyzing of a Solar-Driven CCHP System Based on the First and Second Laws of Thermodynamics

Due to the environmental issues posed by fossil energy sources, reducing the effects of using these resources is one of the main priorities in energy planning today [1]. In this context, in addition to striving for maximum energy savings, the most important concern is to replace these energy resources with renewable ones [2]. Consequently, the use of fuel cell [3], wave [4], wind [5], and solar [6] energies attained substantial attention. Furthermore, wasted heat recovery is another promising approach to save the energy resources [7]. Running the combination of cooling, heat and power systems together, which is commonly named as Combined Cooling, Heating and Power (CCHP) systems [8], is considered as one of the technological suggestions to achieve this aim. However, the noticeable role of other-recently-flourished methods in energy sector, such as Artificial Intelligence (AI) [9], could not be neglected.

Solar energy as a green energy resource, is also employing noticeably for CCHP systems [10]. Sun beams will be trapped using solar tower [11] or solar concentrator [12], then the trapped heat will be transferred to the working fluid of power generator cycle. Indeed, Samiee and Aghdam [13] employed photovoltaic panels to direct electricity conversion in their CCHP which is proposed for cruise ship. They asserted that using hybrid energy resource, the proposed system produced 457.25 kW electricity. The solar driven CCHP system integrated with ammonia driven molten carbonate fuel cell, purposed by Lu et al. [14], shows 57.85% energy efficiency and 60.22%, exergy efficiency. They asserted that, the novel technique they used for heat absorption, enhanced solar power generation by 1.27%. Therefore, the novel system has a higher productivity and economic performance. A hydrogen production sub-system is added to the solar driven CCHP system investigated by Assareh et al. [15], to achieve 90% hydrogen production efficiency. The system cost is reported by $514188.21 per year in case of optimum condition. To achieve sustainable and efficient poly-generation system, Liu et al. [16] investigated a solar assisted CCHP system thermodynamically. During this study, energy and exergy efficiencies are reported by 57.5% and 36.6% under design condition, respectively. Indeed, the carbon emission of investigated system is reported by 0.071 to 0.075 t/GJ.

CCHPs benefit a main power generation part which could be run by a Brayton [17], Rankine [18] or 4-stroke combustion [19] cycle. Therefore, the range of heat sources, wasted heat recovery sub-cycles, and performance improver sub-devices are widely extended [20]. A Brayton cycle is employed as the main power generation part of the solar driven poly-generation system investigated by Georgousis et al. [21]. They optimized the operating condition using multi-objective optimization approach and asserted that the payback period of their system is around 6.4 years while the total efficiency is 55%. They also reported that the exergy efficiency of the studied system could be enhanced up to 23.63% using additional Organic Rankine Cycle (ORC). Using Brayton cycle as the main power section, Zhang et al. [22] reported the Coefficient Of Performance (COP) for their CCHP system by 188.1%. Indeed, Rankine cycle is utilized as the main power generation section in a CCHP system investigated by Cao et al. [23]. Due to the high temperature exhaust flow from common Brayton cycle it is possible to run a Rankine cycle [24], supercritical CO₂ Brayton cycle [25], or ORC [26] as the wasted-heat recovery sub-section. Employing ORC in this case has advantages like, being energetically efficient and environmentally friendly besides enabling investment-savings [27].

Compressed Air Energy Storage (CAES) technology is known as the performance optimizer and enhancer of thermodynamic power cycles [28] as well as CCHP systems [29]. Mohammadi et al. [30] achieved 2.56 kW cooling load, 33.67 kW electricity and 1.82 ton/day hot water with 53.94% trip energy efficiency employing CAES coupled with wind turbine. Arabkoohsar et al. [31] accomplished 30.6, 2.5 and 14.4% exergy efficiencies of cooling, power and heat productions using their suggested configuration of CAES. Yang et al. [32] improved the energy efficiency of the studied CCHP by 1.015% with employing CAES technology and solar energy. Su et al. [33] also reported 77.8 and 3.3% electricity and natural gas saving ratios improvement using synthetic utilization of solar and biogas energy in their studied CCHP. Eisavi et al. [34] stated that the cooling and heating load of CCHP could be raised up to 48.5 and 20.5% employing solar energy.

Looking more detail in investigated literature, it can be asserted that there are lots of efforts all around the world to produce and optimize the renewable electricity using Braytone cycle and the technologies applied on it to enhance the performance shown in Figure 1. However, this approach needs to be improved constantly. The combination of studied approaches and technologies can be considered as one of the possible way to achieve this goal, so in this work, a novel solar-driven CCHP system using the combination of reviewed technologies is proposed for improving the energy efficiency. Then the influence of design parameters on system energy and exergy performance is evaluated in the parametric study to achieve optimal performance. Indeed, such a parametric study could bring the dataset for further AI-based optimization study.

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Figure 1. Solar driven CCHP trend line

For thermodynamically analyzing, each studied component should be considered as a separate control volume. The overall plant is divided into four sub-sections: power generation, refrigerator, heating, and solar systems. Power Generation System (PGS) contains Brayton and Rankine cycles, and the refrigerator is consisted of ORC and compression refrigeration cycle. The following assumption is considered for thermodynamical modeling of proposed system:

-The processes of all devices are assumed as steady-state steady flow (SSSF).

-Kinetic and potential energies and also chemical exergy are negligible.

-The processes of turbines, compressors, and pumps are calculated based on the isentropic efficiency.

The mass and energy conservation and also exergy balance equations in SSSF process for control volume can be written as [3]:

$\sum \dot{m}_{\text {in }}=\sum \dot{m}_{\text {out }}$             (1)

$\dot{Q}_{C . V}-\dot{W}_{C . V}+\sum \dot{m}_{i n} h_{i n}-\sum \dot{m}_{\text {out }} h_{\text {out }}=0$            (2)

$\Delta \dot{\psi}_{\text {sys }}=\sum \dot{\psi}_{\text {in }}-\sum \dot{\psi}_{\text {out }}-\dot{\psi}_D$            (3)

where, $\dot{Q}_{C . V}, \dot{W}_{C . V}, \dot{m}$ and $h$ refer to heat, work, mass flow rates and specific enthalpy in control volume. $\dot{\psi}_{i n}$ and $\dot{\psi}_{\text {out}}$ present the rate of inlet/outlet transferred exergy by heat, work, and mass, and $\dot{\psi}_D$ is exergy destruction. For the SSSF system, the $\Delta \dot{\psi}_{{sys }}$ is negligible, so Eq. (3) could be written as [3, 35],

$\dot{\psi}_Q-\dot{\psi}_W+\dot{\psi}_{\text {mass,in }}-\dot{\psi}_{\text {mass,out }}=\dot{\psi}_D$         (4)

$\dot{\psi}_{\text {in }}-\dot{\psi}_{\text {out }}=h-h_0-T_0\left(s-s_0\right)$          (5)

The solar sub-section contains two parts, namely receiver and heliostat field. The collected worthy heat by the receiver can be computed as [6]:

$\dot{Q}_{\text {Rec,in }}=\eta_{\text {field }} \cdot \dot{Q}_{\text {sun }}=\eta_{\text {field }} \cdot(D N I) \cdot A_{\text {hel }} \cdot N_{\text {hel }}$         (6)

where, $\eta_{\text {field }}, D N I, A_{\text {hel }}$ and $N_{\text {hel }}$ refer to the efficiency of the heliostat field, direct normal irradiance, the area of concentrating and reflecting sun-beams and the number of heliostats. The heliostat field efficiency can be determined as,

$\eta_{\text {field }}=\eta_{\text {cos }} \cdot \eta_{\text {s&b }} \cdot \eta_{\text {int }} \cdot \eta_{\text {att }} \cdot \eta_{\text {ref }}$      (7)

where, $\eta_{\cos }$ refers to cosine effect efficiency; $\eta_{s \& b}$ denotes shading and blocking efficiency; $\eta_{\text {int}}$ presents interception efficiency; $\eta_{{att}}$ accounts for the atmospheric attenuation efficiency and $\eta_{\text {ref}}$ is the reflectivity of the heliostats which are considered 0.83, 0.96, 0.97, 0.93 and 0.88, respectively. The used correlation on the first and second law analyzing for each component are summarized in Appendix 1.

To evaluate each section performance based on first and second laws, pure achieved and consumed energies and exergies should be defined, and the rate of achieved to consumed ones are known as first and second laws efficiencies. For example, in PGS section.

$\begin{gathered}\dot{W}_{\text {net }}=\left(\dot{W}_{G T 1}+\dot{W}_{G T 2}+\dot{W}_{\text {comp } 1}+\dot{W}_{\text {comp } 2}\right)_{\text {Brayton }} +\left(\dot{W}_{G T R}+\dot{W}_{\text {PumpR }}\right)_{\text {Rankine }}\end{gathered}$            (8)

$\eta_I=\frac{\dot{W}_{n e t}}{\dot{Q}_{I H F}}$            (9)

$\eta_{I I}=\frac{\dot{\psi}_{\text {achieved }}}{\dot{\psi}_{\text {consumed }}}=\frac{\dot{W}_{\text {net }}}{\dot{Q}_{I H E}\left(1-\frac{T_0}{T_s}\right)}$   (10)

where, $T_0$ and $T_s$ refer to the ambient and source temperatures near the IHE. The definition of first and second laws efficiencies of each section and overall are reported in Appendix 2.

In this paper, a solar-driven CCHP system is proposed to achieve higher energy efficiency, and a parametric study on compression ratio, and direct normal irradiance were implemented to estimate the impact of design parameters variations on energy and exergy performance of the proposed system. A CAES technology is utilized in the solar-driven Brayton cycle for running in the peak consumption hours, and a Rankine cycle is employed as an axillary cycle for more power generation. Finally, an ORC driven refrigeration cycle and heating system are considered to convert the waste heat of Brayton cycle to the beneficial energy. The main results were listed below:

• 11.75 MW power, 3.2 MW heating load and 6.8 MW cooling load were provided by the proposed CCHP.
• Designed CCHP is able to run 5 hr in compressor deactivated mode passing peak times.
• Proposed CCHP yields energy efficiency of higher than 89%.
• Using proposed CCHP system brings at least 9% Energy saving in comparison with typical-in-use similar systems.
• Studied CCHP system could be considered more green system than the common ones, having less percent of total entropy generation.
• The most exergy destructor component of the proposed system is solar heat absorber by 72% of general system destructed exergy that could be defined as the target for future study.