The objective of this study is to collect energy on the waste heat from air produced by solar ventilation systems. This heat used for electricity generation by an organic Rankine cycle (ORC) system was implemented. The advantages of this method include the use of existing building’s wall, and it also provides the region of energy scarcity for reference. This is also an innovative method, and the results will contribute to the efforts made toward improving the design of solar ventilation in the field of solar thermal engineering. In addition, ORC system would help generate electricity and build a low-carbon building. This study considered several critical parameters such as length of the airflow channel, intensity of solar radiation, pattern of the absorber plate, stagnant air layer, and operating conditions. The simulation results show that the highest outlet temperature and heat collecting efficiency of solar ventilation system are about 120°C and 60%, respectively. The measured ORC efficiency of the system was 6.2%. The proposed method is feasible for the waste heat from air produced by ventilation systems.
The development of the economy and society on a global scale has been accompanied by energy crises and environment pollution, which have become two major problems worldwide. Therefore, the application of renewable energies (solar energy, wind energy, and geothermal energy) in electricity generation is becoming increasingly crucial. In addition, power generation in which the organic Rankine cycle (ORC) is used to recover low-grade energy sources has attracted considerable attention. In recent years, Hung et al. [
Among these sources, geothermal and solar energy are typically used in converting low-grade heat into power and in other applications [
According to the statistical data, air conditioning takes about 40% of electricity demand for building. Therefore, natural ventilation has received considerable attention because it reduces heat gain and induces natural cooling or heating in both commercial and residential buildings and provides potential benefits regarding operational costs, energy requirements, and carbon dioxide emission.
The objective of this study is to collect the waste heat from air produced by ventilation systems. The waste heat was used for electricity generation in which an ORC system was implemented, as shown in Figure
Schematic diagram of the ventilation in building combined with ORC system.
A wide variety of natural ventilation systems are presented in the literatures. Solar wind towers [
In this study, we applied experiments and CFD to solve problem of solar storage wall combined with the organic Rankine cycle. The fluid flow and heat transfer of this solar storage wall were analyzed numerically, and the results were validated based on the experimental data. Subsequently the parameters, such as the thickness of the air gap, the operating conditions, and height of flow, that clearly influence collector efficiency were analyzed. These results provide a reference for the future design and optimization of solar storage wall.
In this section, we present the details of the experiments and performance analysis conducted in this study.
Factors such as time, season, and weather cause the experimental results to become unstable. Therefore, we constructed a solar simulator that can provide stable energy on an absorber plate, as shown in Figure
Distribution of heat flux on glazing for solar simulator.
Solar simulator
Measurement location
Heat flux (W/m2)
The collector module consisted of glass, an absorber plate, an airflow channel, a stagnant air layer, insulation, and an aluminum collector frame used to install the components, as shown in Figure
Schematic diagram of the experimental setup and view of the solar chimney.
Experimental setup
View
The air inlet to the chimney was located at the bottom of solar storage wall and the air outlet was located at the top. Inlet airflow was collimated by employing a laminated array to provide the velocity component in the
The variables measured in this experiment included the inlet and outlet air temperature, ambient temperature, and the solar radiation and mass flow rate of the air. The collector was instrumented with three T-type thermocouples (
The useful energy gained using the solar ventilation system can be expressed as [
The general test procedure involves determining
In this study, commercial CFD code ANSYS FLUENT [
In the ventilation system, the study used rectangle pipe flow, and the hydraulic diameter,
Continuity equation:
Momentum equation:
Energy equation:
The discrete ordinates (DO) model was chosen for conducting the simulation of radiation heat transfer. This was achieved by coupling radiative transfer equation (RTE) and the convective energy equation [
The complete geometry of the system is divided into three sections: the entrance, test, and exit sections. The 3D domain used for CFD analysis was set up to have a height of 1.9 m, width of 0.6 m, and a depth of 0.04 m. The depth of the stagnant air layer and airflow is 0.18 m and 0.02 m, respectively. To reduce the burden of computational time and memory space on the computer, the system under consideration was assumed to have a symmetric plane.
The 3D computational domain of the solar ventilation system was created and meshed according to its actual size by using Gambit [
Computational model and boundary conditions.
Uniform air velocity was introduced at the inlet while a constant pressure was applied at the outlet. The system under consideration was assumed to have a symmetric plane. In addition, the collector frame was assumed to be adiabatic. The heat loss from the back plate and the glass to the surroundings was considered, and no-slip boundary condition was assigned to the walls in contact with the fluid in the model.
The properties of the working fluid (air), absorber plate (aluminum), and glass material were assumed to remain constant. The glass and absorber plate were homogeneous and isotropic. The thermal conductivity of the frame, absorber plate, and glass was temperature independent. The thermophysical properties of the working fluid, glass, and absorber plate are listed in Table
Thermophysical properties of working fluid (air) and absorber plate (aluminum) for computational analysis.
Properties | Working fluid (air) | Absorber plate (aluminum) | Glass |
---|---|---|---|
Density, |
Incompressible ideal glass | 2719 | 2500 |
Specific heat, |
1006.43 | 871 | 750 |
Viscosity, |
1.7894 × 10−5 | — | — |
Thermal conductivity, |
0.0242 | 202.4 | 1.4 |
Absorptivity, |
— | 0.9 | 0.12 |
Emissivity, |
— | 0.4 | 0.9 |
After establishing all the relevant settings, ANSYS FLUENT performed the calculation in an iterative manner until a sufficient tolerance, defined by the user, was achieved. The convergence criterion of 10−6 represents the residuals in the continuity equation; 10−3 for the residuals of the velocity components and 10−6 for the residuals of the energy were assumed.
We investigated the feasibility of solar ventilation in natural convection and compared the measured and calculated results. The results of solar simulator show that the uniform heat flux can be assumed as 800 W/m2. The heat transfer coefficient of the wind and temperature of ambience were assumed to be 20 W/m2-K and 30°C, respectively. The error of the temperature between the CFD and experimental results is given by
Figure
Air temperature comparison between measured and calculated results under free convection.
The flow field and temperature distribution in the computational domain.
In this study, the maximal temperature and mass flow rate were determined using various lengths of airflow under ideal conditions for the proposed system. Figure
The maximum temperature and mass flow rate with various lengths of airflow.
The accuracy of the calculated numerical results was accepted. Therefore, the experiment and simulation were applied to obtain optimal geometric design. CFD provides numerical advantages over experiment-based approaches, such as the substantial reduction in human labor and work time and the costs of materials. This study investigated the influence of various lengths of airflow, operating conditions of the inlet, the energy of heat flux, the type of absorber used for achieving efficiency, and the width of the air layer.
According to the results described in Section
Outlet temperature and efficiency of the collector versus mass flow rate at
The measurement results and CFD simulation, in which forced convection
Temperature comparison between measurement and CFD calculation at
The temperature distribution in the exit section of solar ventilation.
The longer the residence time in the airflow channel, the smaller the temperature difference between the aluminum and the air. Thus, the heat transferred to the air via the aluminum is reduced. If the air does not very smoothly flow in and out through the flow channel, it may cause a high percentage of heat returned to the environment by way of thermal radiation.
The experimental study was conducted on the Penghu Islands. The Penghu Island of Taiwan was located at 23°28′17′′ latitude north and 119°30′45′′ longitude west. Figure
Photo of the solar chimney.
Surface temperature variation at different location and weather condition (September 18, 2013).
Maximum temperature of system and solar irradiation
Velocity (m/s)
Regarding heat transfer, a large absorber area may result in a substantial increase in temperature during solar ventilation. Therefore, potential enhancement of heat removal was investigated by the changing length of airflow and solar radiation, and
Effect of the lengths of airflow channel and solar heat flux by CFD simulation at
Outlet temperature
Efficiency
Furthermore, the intensity of solar radiation depends on the orientation of the solar ventilation system, the day of the year, and the hour of the day, among other working conditions. Therefore, this study investigated the effect of solar radiation, and the results are shown in Figure
The convective heat transfer rate in the airflow channel can be augmented by increasing the heat transfer surface area and increasing the turbulence inside the channel. This study investigated the effect of various types of absorber plates which are inexpensive and common. Figure
Three types of absorber plate in the solar ventilation.
Figure
Temperature difference and efficiency of the collector versus mass flow rate for various absorber plates by experiment measurement.
Temperature difference
Efficiency of the collector
Heat loss occurs in the air space between the glazing and absorber plate through convection and conduction back the atmosphere. To reduce the heat loss of the absorber plate, the influence of various widths of the stagnant air layer and the associated heat transfer coefficient of wind on thermal performance was investigated. Five air layer widths were used in the CFD simulation: 0.3, 0.8, 1.8, 2.8, and 3.8 cm. The heat transfer coefficients were 10, 20, and 30 W/m2-K, respectively.
The various air layer widths and wind velocity are shown in Figure
The outlet temperature for various widths of air layer and heat transfer coefficient of wind by CFD simulation.
In this section, we investigated the economic assessment for solar storage wall combined with the organic Rankine cycle, as shown in Figure 1 → 2: compression (working fluid feed pump); 2 → 3: heat supply (solar ventilation system); 3 → 4: expansion (expander); 4 → 5: heat rejection (condenser).
Schematic
At the same time, the process 3 → 4a which appeared in Figure
From above, result indicates that the highest outlet temperature and efficiency of air collected were approximately 100°C and 60% (with 4 m length of air channel and absorber plate of Case I). In this study, heat exchanger’s performance was assumed as 80%, and the temperature of hot water was approximately 80°C. Figure
Energy and outputs of the experimental cycle.
Properties | Working fluid (air) |
---|---|
Area of collector, m2 | 48 (8 m × 6 m) |
Solar radiation, kW/m2 | 0.80 |
Efficiency of collector, |
60% |
Energy, kWh/day (8 hours/day) | 184.3 |
Efficiency of Rankine cycle, |
6.2% |
Power generated, kWh/day | 11.43 |
An innovative application of ORC systems combined with solar ventilation was investigated and an experimental prototype was designed, constructed, and tested in this study. Experiments and CFD simulations were used to investigate the performance of the proposed solar ventilation system. The results are summarized as follows. The deviation of the solutions obtained using CFD simulations was generally within the acceptable range, which proves that CFD is a qualified tool for predicting the behavior and performance of a ventilation system. The thermal performance and outlet temperature of air, when a slight forced convection was supplemented, were superior to those produced when a pure natural convection was considered. An optimal configuration of a solar ventilation system has a stagnant air layer of 2 cm, an air channel length of 4 m, and an absorber plate of Case I, and the optimal range of air inlet for thermal performance of a collector is approximately 2-3 m/s. The proposed method is feasible for the solar heat collected by air ventilation systems, and the heat was used for electricity generation in a system incorporated with ORC.
Numerical and experimental methodologies are treated. An innovative concept of combining the waste heat of ventilation with ORC system is introduced. Various types of solar ventilation system are discussed.
Area (m2)
Specific heat (J/kg-K)
Hydraulic diameter of duct (m)
Collector heat removal factor
Height of absorber plate (m)
Enthalpy (kJ/kg-K), heat transfer coefficient (W/m2-K)
Intensity of solar radiation (W/m2)
Thermal conductivity (W/m2-K)
Prandtl number
Mass flow rate (kg/s)
Wetted perimeter
Useful energy gain (W)
Temperature (K)
Top loss coefficient
Velocity (m/s)
Power (kW).
Absorptivity
Effective transmittance-absorptance product
Density of material
Emissivity
Viscosity
Turbulent viscosity
Molecular thermal diffusivity
Turbulent thermal diffusivity
Thermal efficiency
Collection efficiency of the solar collector
Rankine cycle efficiency
Overall efficiency of the system
Different
Error of temperature.
Ambient
Solar ventilation
Inlet
Outlet
Rankine cycle
Wind
Turbulent.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research work has been supported by the National Science Council, Taiwan, under Grant of Contract nos. NSC 103-3113-P-007-002 and NSC 101-2221-E-027-039.