Solar energy powered organic Rankine cycle vapor compression cycle (ORC-VCC) is a good alternative to convert solar heat into a cooling effect. In this study, an ORC-VCC system driven by solar energy combined with electric motor is proposed to ensure smooth operation under the conditions that solar radiation is unstable and discontinuous, and an office building located in Guangzhou, China is selected as a case study. The results show that beam solar radiation and generation temperature have considerable effects on the system performance. There is an optimal generation temperature at which the system achieves optimum performance. Also, as a key indicator, the cooling power per square meter collector should be considered in the hybrid solar cooling system in design process. Compared to the vapor compression cooling system, the hybrid cooling system can save almost 68.23% of electricity consumption.
The energy consumption used for air-conditioning has increased drastically in recent years [
Thermally activated cooling technologies have been reported by many researchers [
The solar ORC-VCC systems convert solar heat into cooling effect, accomplished by utilizing a Rankine cycle to generate the shaft work to drive a vapor compression cycle. The ORC-VCC provides an alternative to solar cooling besides absorption and adsorption cooling. However, due to the unstable and discontinuous characteristics of solar energy, the back-up source or heat storage equipment are required to ensure smooth operation of solar cooling system, resulting in a high engineering cost. To solve this problem, a hybrid solar energy and electric motor cooling system is proposed in this study. In the hybrid cooling system, solar energy powered ORC and electric motor are operated in parallel, and the problem of unstable and discontinuous energy supply for solar energy can be solved by using electric motor even solar collectors do not work on rainy days. The main aim of this study is to evaluate and analyze the performance of an office building air-conditioning using the ORC-VCC system driven by solar energy combined with electric motor. R245fa is selected as the working fluid and its thermophysical properties are listed in
Fluid type | Molecular mass (kg/kmol) | Critical temperature (°C) | Critical pressure (kPa) | Normal boiling point (°C) | Ozone depletion Potential | Global warming Potential |
---|---|---|---|---|---|---|
Dry | 134.05 | 154.01 | 3651.00 | 15.14 | 0 | 950 |
The schematic diagram of the solar ORC-VCC system is presented in
To yield high system efficiency, the parabolic trough collector is selected to collect solar energy. The R245fa absorbs the solar energy collected by the parabolic trough collector and is vaporized to saturated or superheated vapor in the solar collector. The high pressure vapor drives an expander attached to a vapor compressor. The exhaust working fluid from the expander is condensed into liquid in the condenser 2 by the heat sink. The liquid R245fa is then pumped back to the solar collector. For VCC, the vaporized R245fa from the evaporator flows into compressor and becomes high pressure vapor through compression process. And then the high pressure vapor is cooled down in the condenser 1. Through throttle valve, the liquid R245fa evaporates in the evaporator and generates refrigeration effect. In addition, a frequency conversion motor is used to generate shaft work to drive the compressor. Solar powered ORC and the variable-frequency motor are connected in parallel, and the variable-frequency motor will start when the cooling needs for building cannot be satisfied completely by solar energy.
To simplify the mathematical model, some assumptions have been made as follows:
Steady-state conditions are considered.
Heat and friction losses in the system are neglected.
Working fluid leaving the generator, evaporator and condenser is saturated.
Based on these assumptions and referring to the system presented in
where
The power consumption of the working fluid pump is expressed by:
where the subscript “pp” represents working fluid pump.
The useful energy gained from the solar collector, i.e., generator, is calculated from:
where
The net power output of the ORC is given by:
where the subscript “net” represents the net power.
The thermal efficiency of the ORC system is defined as follows:
Note that the shaft of the expander in the ORC and the compressor in the VCR cycle are directly coupled. The output power of the expander is equal to the input power of the compressor, i.e.,
where the subscripts “com” and “VCC” represent the compressor and VCC subsystem, respectively.
The cooling power produced from the solar energy is expressed as:
where the subscript “eva” indicates evaporator.
The coefficient of performance of the VCC is given by:
The coefficient of performance (COP) of the ORC-VCC is given by:
The thermal efficiency of the solar collector is determined by:
where
The overall thermal efficiency of the ORC-VCC driven by the solar energy is given by:
When the cooling power produced by the solar driven ORC-VCC cannot meet the cooling load needed, the electric motor will start working for compensation, which is given by:
where “em” denotes the electric motor.
The electric consumption by the motor is determined by:
The cooling power per square meter collector is calculated as follows:
where
The generation temperature (outlet temperature of solar collector) is set at the range of 80–140°C, the condensation temperature is at 40°C, and the evaporation temperature keeps at a constant temperature of 5°C. The isentropic efficiencies for working fluid pump, expander and compressor are 0.90, 0.85 and 0.80, respectively, and the electric motor efficiency is 0.96. The performance of ORC-VCC driven by solar energy is firstly analyzed and evaluated. And then a case study of office building air-conditioning using ORC-VCC driven by solar energy combined with electric motor is conducted.
To evaluate the thermodynamic performance of ORC-VCC driven by solar energy, the intensity of beam solar radiation is set at 600 W/m2.
Based on the above result of analysis, it can be concluded that there is the optimal generation temperature, at which the ORC thermal efficiency, COP for ORC/VCC, overall efficiency of solar cooling system and cooling power per square meter collector achieve the maximum values. Therefore, the cooling power per square meter collector should deserve more attention while in the design of the hybrid solar driven cooling system.
An office building located in Guangzhou, China is selected as the study subject, and its total construction area is 40000 m2. The air-conditioning system of office building works from 9:00 to 17:00 every day and its cooling load versus time is depicted in
The variations of
The variations of
In this paper, an ORC-VCC system driven by solar energy combined with electric motor is proposed to ensure smooth operation under the conditions that solar radiation is unstable and discontinuous. With an office building located in Guangzhou, China selected as a case study, a thermodynamic model is built to investigate the thermodynamic performance of the hybrid cooling system. The following conclusions can be drawn:
The generation temperature has a considerable influence on the thermodynamic performance of solar ORC-VCC system, which in general increases initially and then decrease with the increased generation temperature. Considering the thermo-economic performance of solar ORC-VCC system, more attention should be paid to cooling power per square meter solar collector.
With the addition of electricity motor, the hybrid solar cooling system can operate stably to meet the cooling load for the target building even when solar energy is unstable and discontinuous. The electricity consumption of electric motor is more sensitive to the solar radiation intensity and cooling load. As for the case study, thanks to the solar ORC-VCC system, the electricity consumption saved reaches up to 8094.87 kWh per day.
Moreover, the present paper only focuses on the thermodynamic performance characteristics of solar ORC-VCC under the design conditions. In the future, an off-design analysis should be conducted for reliable and cost-effective operation. Besides, the addition of electricity motor puts forward higher requirements for control strategy research. Also, the power matching and regulation characteristics study should be performed.
collector area, m2
coefficient of performance for VCC
coefficient of performance for ORC/VCC
direct radiation intensity, W/m2
enthalpy at expander inlet, kJ/kg
enthalpy at expander outlet based on isentropic process, kJ/kg
enthalpy at working fluid pump inlet, kJ/kg
enthalpy at working fluid pump outlet, kJ/kg
enthalpy at working fluid pump outlet based on isentropic process, kJ/kg
enthalpy at evaporator outlet, kJ/kg
enthalpy at compressor outlet based on isentropic process, kJ/kg
enthalpy at evaporator inlet, kJ/kg
mass flow rate for VCC, kg/s
mass flow rate for ORC kg/s
pressure at compressor inlet, kPa
pressure at compressor outlet, kPa
generator heat input, kW
cooling power from electric motor, kW
cooling power from solar energy, kW
total cooling power from office building, kW
cooling power per square meter collector, W/m2
generation temperature in the generator, °C
condensation temperature, °C
compressor work input from ORC, kW
electric motor work input in ORC/VCC, kW
expander work output, kW
working fluid pump power consumption, kW
net work output for ORC, kW
electric motor work input without solar energy, kW
compressor isentropic efficiency
electric motor efficiency
expander isentropic efficiency
organic Rankine cycle efficiency
working fluid pump isentropic efficiency
thermal efficiency of solar collector
overall efficiency of solar cooling system
temperature difference of inlet and outlet of collector, °C