1. Introduction
Renewable energy utilization is an important weapon for tackling critical energy problems such as fossil fuel depletion, global warming, increasing energy demand and increasing electricity prices [
1,
2,
3]. Furthermore, trigeneration and polygeneration systems are highly efficient units that can produce numerous useful outputs simultaneously [
4,
5]. Thus, the concept of using renewable energy sources to feed polygeneration systems is a viable and environmentally friendly solution for future sustainable systems. Another important aspect of these systems is their use of environmentally friendly working fluids, which are usually natural fluids such as CO
2, NH
3, propane and butane [
6]. However, these fluids present some limitations, which are related to performance, toxicity and flammability. CO
2 seems to be the most promising fluid as it can operate both in transcritical and supercritical configurations and it is not toxic, not flammable and it is a cheap working fluid [
7]. Moreover, there are alternative working fluids (not natural) that have zero ODP and a not so high GWP (<1000) that can be used, like R32, R245ca, R245fa, R365mfc and R1336mzz [
8].
In the literature, there are many studies that examine polygeneration systems driven by renewable energy sources (solar, geothermal, biomass and wind) [
9]. Al-Sulaiman [
10] studied a system with an organic Rankine cycle (ORC), absorption heat pump and parabolic trough solar collectors, which presented a 20% exergy efficiency. A similar configuration was examined and optimized by Bellos and Tzivanidis [
11], who found a global maximum exergy efficiency of 29.4% with an energy efficiency of about 150%. A trigeneration system driven by solar energy was studied by Eisavi et al. [
12]. This system incorporated an ORC and a double-stage absorption heat pump and had 13% exergy efficiency and 96% energy efficiency. Mathkor et al. [
13] investigated a unit for fresh-water, cooling and electricity production that was driven by a parabolic trough collector, which had an exergy efficiency of 42%. In another interesting work, Voeltzel et al. [
14] examined a cogeneration unit for cooling and electricity production based on absorption technology. They conducted an experimental investigation and found the maximum electricity production was up to 0.7 kW, while the maximum cooling production was up to 8 kW.
The combination of solar and geothermal energy has been studied by Khalid et al. [
15]. They studied a configuration with an ORC and absorption chiller, which had 76% energy efficiency and 7.3% exergy efficiency. Bellos et al. [
16] studied a polygeneration system driven by solar energy and biomass. This system produced heating at two temperature levels, cooling and electricity. The energy efficiency was 51.3% and the exergy efficiency was 21.8%. Harrod et al. [
17] studied the use of a trigeneration system with a Stirling engine as the primary mover, which was driven by biomass heat input. They found that the recovery of the engine waste heat was a promising idea for improving the system’s sustainability.
The next part of our literature review focused on trigeneration systems that use CO
2 as the working fluid. Wang et al. [
18] studied a Brayton power cycle with an ejector device for power and refrigeration production, which also produced heating from the turbine outlet heat. The system was driven by compound parabolic solar collectors and it showed 28.8% exergy efficiency and 53% energy efficiency. Xu et al. [
19] studied the modification of the previous system with extraction, which resulted in a 22.5% exergy efficiency enhancement. Hou et al. [
20] studied a complex cycle with CO
2 as the main working fluid. This system was driven by methanol and it includes a typical air-Brayton cycle, which feeds heat to a CO
2 cycle for power and refrigeration. The heating produced by this system is practically steam, which is created by the waste heat of the air-Brayton cycle. The system is optimized exergo-economically and the exergo-economic factor was found to be 28.23%. In another work, Misha and Singh [
21] examined a trigeneration unit with parabolic trough solar collectors, supercritical CO
2 Brayton cycle and a bottoming absorption cycle. They found that the optimum maximum temperature occurred at 650 K and in this case, the exergy efficiency was about 75% and the energy efficiency was about 45%. Balafkandeh et al. [
22] optimized a biomass-driven trigeneration system with a CO
2-Brayton cycle and an absorption chiller. They found maximum energy and exergy efficiencies at 43.7% and 47.8%, respectively. Interestingly, Fan et al. [
23] studied a trigeneration system with a supercritical CO
2 recompression Brayton cycle and a Rankine cycle with an ejector device. This unit was fed by heat from a nuclear reactor and it was optimized with a multi-objective optimization procedure. The exergy efficiency was found to be up to 69% and the trigeneration configuration had 9.2% higher exergy efficiency compared to the system with supercritical CO
2, which produced only electricity. Additionally, an interesting optimization was performed by Yang et al. [
24] for a CO
2-based trigeneration system. They found that the optimum cooling to electricity ratio had to be in the range of 1.37 to 1.53. Lastly, Zare and Takleh [
25] studied a geothermal-based trigeneration system with CO
2. They studied two systems with a modified Brayton cycle with an ejector inside the system. They found that the use of an internal heat exchanger in place of the gas cooler led to enhancement in the performance and to a maximum exergy efficiency of 32%. Finally, the use of the CO
2 transcritical cycle for storage issues can be applied in polygeneration systems as an alternative choice as has been suggested by Ayachi et al. [
26].
Our literature review indicates that there is a lot of interest in trigeneration and polygeneration energy systems with renewable energy sources and natural refrigerants. In this direction, the present work investigates a novel polygeneration system for refrigeration, power and heating production at two temperature levels. This system includes a thermodynamic cycle that is practically, a combination of a mechanical compression power cycle and a recompression Brayton cycle. The unit is fed by biomass and the working fluid is CO
2. To our knowledge, there are no other studies that have investigated a system such as the present one, which combines four useful products, a renewable energy source as the heat input (biomass in this case) and a natural refrigerant as the only working fluid. Thus, this work suggests a totally new, and source configuration. The system was examined in steady-state conditions and it was analyzed parametrically. The study was conducted by using a developed thermodynamic model in Engineering Equation Solver [
27].
2. Material and Methods
2.1. The Examined System
In this work, a polygeneration system was investigated as shown in
Figure 1. A biomass boiler was used as the energy heat input and it gives the proper energy for the system operation. The working fluid in the examined thermodynamic cycle is CO
2, which is an environmentally friendly fluid. This fluid has a relatively low critical point with the critical pressure being 73.8 bar and the critical temperature being 31.1 °C.
Refrigeration is produced from the evaporator and saturated vapor (quality equal to 100%) is produced (
Figure 1, state point 1). This quantity is compressed from low pressure to medium pressure with the use of the compressor (state point 2). After this, a proper heat exchanger is used in order to produce low-temperature heating at 45 °C and state point 3 is selected as 50 °C in all the cases. Practically, a temperature difference of 5 °C [
28] is used in order to ensure proper heat transfer. Moreover, a gas cooler is used in order to reject heat to the ambient and so the CO
2 reaches state point 4. A second compressor re-compresses this quantity up to state point 5. The present configuration has intercooling in order to reduce the compressing work and to recover heating. Moreover, the recompression from state point 4 to state point 5 needs a small amount of work because state point 4 is close to the critical point of the CO
2 (from the supercritical side) and so the density of the CO
2 is relatively high, which reduces the pumping work.
After the second compressor, there is a heat exchanger in order to preheat the CO
2 before the biomass boiler. So, the CO
2 is warmed up to state point 6 and goes into the boiler where its temperature is increased up to the turbine inlet temperature (TIT) at state point 7. The turbine receives fluid of high pressure at state point 7 and expands it up to state point 8 (medium pressure), and so, work is produced. The heat exchanger reduces the temperature of the supercritical CO
2 (state point 9) and the next step is the heat recovery in the high-temperature levels (state point 10). A pinch point of 5 °C was also applied in this device, and when the high heating temperature is 80 °C, for example, the temperature level in state point 10 is 85 °C in this case [
28]. The gas cooler follows the heating device and heat is rejected to the ambient up to state point 11. A throttling valve is used to reduce the pressure from the medium to a low level. This device is assumed to be adiabatic, and so, the enthalpy is the same in its inlet and outlet. The state point after the valve (12) is the inlet in the evaporator and so the cycle closes.
A general overview of the present system can be performed by explaining the heat exchanges of the examined system with the ambient and the external sources. The examined system takes heat from a biomass boiler (Qb) at a high temperature (e.g., 700 °C) and there is a heat input in the evaporator (Qref), which is the refrigeration load in the lowest cycle temperature (e.g., 5 °C). The system rejects heat to the ambient in the gas coolers (Qgc,1 and Qgc,2) as it has a relatively low temperature of 35 °C, and so these heat rates are rejected and not reused. There are also the heating heat exchangers, which perform energy recovery, and consequently, heating production. More specifically, the (Qheat,low) is produced at 45 °C, while the (Qheat,high) is produced at a higher temperature (e.g., 80 °C). The compressors consume a part of the work that the turbine produces, while there is a net-work production from the system to the grid.
It should be noted that this study is a thermodynamic work and some parameters have been selected to have their ideal values. In this direction, the mechanical efficiencies were selected to be 100% and the heat exchange modeling from the heat exchanger to the ambient and to the heating production were done without taking the overall heat transfer coefficient into consideration.
An alternative design could be with a medium pressure lower than the critical pressure of the CO2, and in this case, the heat rejection in the ambient could be conducted with a condenser and at a lower temperature. However, this configuration would only operate in relatively cold conditions with an ambient temperature up to 25 °C. Thus, this work rejected heat in a supercritical pressure in order to avoid having restrictions and the system was designed to take advantage of this section by using the proper heat exchanger devices. Moreover, another option is to couple the turbine with the compressors in the same shaft in order to improve the overall performance of the systems, to reduce the cost and to develop a compact design.
2.2. Mathematical Formulation
This section includes the basic mathematical equations for the simulation of the suggested system. These equations are developed by the energy balance in the various devices. Firstly, the equations for the determination of the energy inputs/outputs are given:
The heat input (
Qb) in the system can be written as:
The useful heat input in the system is written by using the energy balance in the boiler and with the use of the boiler efficiency (
ηb):
The refrigeration production (
Qref) in the evaporator is written as:
The low-heating production is calculated as:
The high-heating production is calculated as:
The turbine power production (
Ptur) is given as:
The low compressor consumption (
Pcom,low) is given as:
The high compressor consumption (
Pcom,high) is given as:
The net power production of the system (
Pnet) is calculated as:
The system energy efficiency (
ηen) is given as:
The system exergy efficiency (
ηex) is given as:
In the previous equation, the temperature levels are in Kelvin units and the reference temperature (T0) is selected as 298.15 K.
Moreover, the energy balance in the gas coolers can be written as below. The subscript “1” corresponds to the gas cooler between the two compressors and the subscript “20” indicates the gas cooler after the turbine.
The throttling valve reduces the CO
2 pressure and this device is assumed to be adiabatic. This fact makes the process isenthalpic and more specifically:
The heat transfer in the heat exchanger can be described by the energy balance between the two streams and the heat exchanger’s effectiveness.
The process in the turbine is described by the isentropic efficiency which is defined as below:
For this work, the following formula was used to calculate the turbine isentropic efficiency [
29]:
The process in the low-pressure compressor is described by the isentropic efficiency, which is defined below for the two compressors:
The following formula was used for calculating the low-pressure compressor isentropic efficiency [
29]:
The process in the high-pressure compressor is described by the isentropic efficiency, which is defined below for the two compressors:
The following formula was used for calculating the high-pressure compressor isentropic efficiency [
29]:
The selected formulas for the isentropic efficiencies for the compressors and the turbine are frequently chosen in the literature on CO
2 as the working fluid [
29].
2.3. Methodology
In this study, the polygeneration system was investigated in steady-state conditions with a developed model in Engineering Equation Solver (EES) [
27]. The presented equations of
Section 2.2 were used in the simulation of the present study. Treated wood was used, with a lower heating value (H
u) equal to 15,290 kJ kg
−1 [
30]. More specifically, the as-received fuel had 14.6% moisture, 4.44% ash, 41.68% C, 4.88% H, 33.39% O, 0.99% N and 0.07% S.
Table 1 shows the main data for the configuration that was examined. The system was firstly examined in nominal operating conditions (
Section 3.1) and also it was studied parametrically (
Section 3.2). The nominal values of the reference scenario are given in the table, and in all the cases, the heat input was constant at 100 kW. The parameters examined in the parametric study are the following: turbine inlet temperature, high pressure, medium pressure, heat exchanger effectiveness, refrigeration temperature, heat rejection temperature and high heating temperature. Moreover, the temperature level in the turbine inlet can be 700 °C or higher according to the literature [
31].