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Innovative thermo-solar air heater

In the present work we elaborate the innovative design of the solar air heater and justify it by a Computational Fluid Dynamics (CFD) simulation, implementing and experimentally testing a sample. We propose to use this device for maintenance of constant ambient conditions for thermal comfort and low energy consumption for indoor environments, inside greenhouses, passive houses, and to protect buildings against temperature fluctuations. We tested the functionality of our sample of the solar air heater for 50 weeks and obtained an agreement between the results of the numerical simulation, implemented using OpenFOAM (an open source numerical CFD software) and the experimental results.


Introduction
Heating, ventilation and air-conditioning (HVAC) systems are mainly designed for the building sector aiming to ensure the comfort living standards for various climatic zones. The building sector accounts for more than 39% of the primary energy requirements [1] and is a main contributor to carbon emission.
The Solar-HVAC has been considered as an alternative to reduce contribution of the primary energy and in this respect, are developed solutions based on the direct transformation of the solar energy in internal energy of the transport medium [2,3,4,5].
This basic principle is successfully applied to the solar air heaters in passive houses keeping a minimal comfortable temperature of 15 o C [6] during cold seasons.
One niche where the solar-air heaters can bring an input of additional heat during cold season is the greenhouse growers. The cost of fuel is an increasingly significant production expense for greenhouse growers in temperate climates.
High heating costs motivate growers to improve the efficiency of crop production to minimize energy inputs. The two parameters influencing plant development are: mean daily/night temperature (MDNT) and photosynthetic daily light integral (DLI) [7].
MDNT usually must stay in a close range of 15−20 o C for optimal conditions in the greenhouse environment. During warm season the growers are faced with high energy consumption for seeds and fruits drying, the intermediate stage in preservation. Several experimental demonstrations and reviews show that the electricity from primary energy sources can be replaced with solar energy (thermal and photovoltaic [8,9,10] conversion followed by storage in heat and electricity) [11,12,13].
To date, the key technologies applied to greenhouses are focused mainly on transforming diurnal solar energy into heat storage complemented with a smart insulation. The proposed solutions of solar energy utilization and the reduction of heating power via various combined systems (external thermosolar systems coupled with a storage tank, heating pumps and photovoltaics) [11] show promising solutions but the initial investment with return of investment are visible in the price of vegetables that can be higher than when using classical electricity heating.
Another key issue until now not taken into account is the solar energy conversion, during daylight, to internal energy of the air from greenhouse with free 2 convective circulation. Given how it is not appropriate to use forced convection in a greenhouse because the airstreams have a negative effect on the plant growth [14], the only solution remains the free convection for thermal energy transfer.
A second requirement is to maximize the heat transfer from a blackened flat surface under sunlight irradiation to the backside cavity. In this respect, the solution proposed with this device is to increase the backside surface of the solar absorber [15] from A to nA by one set of decorative elements uniformly distributed along the air stream direction. If in an air-heater with forced convection such elements can induce a transition from laminar to turbulent flow decreasing the efficiency of the thermal energy transfer [16,17] in the air-heater with free convection such elements have a minimum effect only in the hydrodynamic resistance [18,19]. The maximum heat transfer in the free convective air heaters is dependent only on the residence time with the hot surface [20].
Taking into account the above considerations we propose one cost-effective solution for one solar air-heater to supply thermal energy by free convection of the air into a greenhouse as well as for other farming application [21], as for a supplementary advantage of the air heaters which work upon the principle of natural convection is their independence from external energy sources.
By comparison with other types of solar air systems glazed [22,23], unglazed [24,25], and with double glazing and double pass air circulation [26] for a higher yield and relative simple production was chosen the simple glazing design with an effective back cavity. To ensure maximal conversion of solar radiation and efficient heat transfer to air flow the solar absorber was made from blackened aluminum. This decision was justified by the fact that, usually, most energy losses occur through the front cover [26], other parts being insulated.
One of the advantages of portable air heaters is the possibility to flexibly vary the power of the heating in respect to the consumer's preferences and weather.
In this paper, we propose a new model of a portable, light-weighted and modular air heater based on solar air heating and natural convection inside of the device shown in Figure 1, which is suitable to be integrated in greenhouses, passive houses and office buildings, and to dry fruits, seeds, and nuts [27,28] during warmer seasons.

Device description. OpenFOAM simulation
Solar air heater design. The most effective and flexible air heaters are made as self-contained devices, meaning that they can be attached to an exterior wall of a building [29,30]. A vertical installation is considered. The unit consists of an insulated frame (implemented out of extruded polystyrene foam), a solar absorbent (presented by a blackened aluminum metal board inside of the thermo-insulated case), a front plexiglass glazing, an inlet for the incoming air recirculation, and an outlet. At the outlet of the heater is placed a hood in order to collect hot out-coming air (Figure 2 a).
We propose an innovative way of leaving a cavity between the back insulating part and the solar absorbent in order to decrease dissipation of thermal energy.
This can lead to an efficiency increase by 30% in comparison to the glazed single pass solar collector, especially in case of air recirculation. In other solar air heaters a part of the thermal energy losses occur due to the contact between hot air and cold front glazing, which we have successfully prevented in our device. For better efficiency, we designed the solar absorbent part to have a flat side that faces the glazing, and the radiator-like surface facing the insulating back part. At the back part of the blackened aluminum board with a thickness of 0.8 mm U-shape profiles are made with the dimensions of 7.5 mm to increase the contact surface between heated air and the metal (Figure 2

b).
Due to the fact that the heat transfer is three times larger at the back side of the metal board in comparison with the front side area [31,15], we have: Theoretical model. the functionality of the solar air heater is achieved by the buoyancy phenomena [32,33] and greenhouse effect [34]. Solar rays enter the plexiglass glazing and are captured by the solar absorbent. Due to that, the air inside of the installation is heated up. Because of the thermal transfer the 4 air inside of the back cavity is heated up. Temperature differences between the exterior air and the air inside of the heater lead to the appearance of buoyancy force, by which the incoming air is pushed up through the heater.
The decreasing air density because of the heating results in a positive buoyancy force. The greater the thermal difference and the height of the structure, the greater the buoyancy force. At the same time, the maximum value of the velocity is limited by the air friction. The hot air exits the device at the outlet and can be collected for future consumption by a hood.
The airflow used to calculate the efficiency of the air heater can be calculated by the formula: Where S is the surface area of the hood and u out and ρ out = ρ in Tin Tout are velocity and density at the output of the air heater, T out and T in are absolute temperatures at the outlet and inlet of the device respectively.
The thermal performance of the air heater can be calculated by the formula: where c air is specific heat of air, ∆t is the difference between inlet and outlet temperatures, W solar is solar flux measured by pyranometer and θ is incidence angle.
OpenFOAM numerical simulation. The processes inside the solar converter are strongly nonlinear and cannot be evaluated analytically. For the simulation (k − ω) RANS (Reynolds Averaged NervierStokes) turbulence model and Buoyant Simple Foam solver were used, which according to [38] gives satisfactory results. It is a steady state solver, which uses ideal gas approximation for air convection for buoyant and turbulent flow of compressible fluids for ventilation and heat transfer.
The equations used by Buoyant Simple Foam solver are: The h is the specific enthalpy; u is velocity, α is air thermal diffusivity, and µ is dynamic viscosity.
The simulation was stopped when the residuals became constant with values less than 10 −6 for temperature and pressure and 10 −5 for velocity.
The simulation was repeated for different heights of the heater within parameters from 1 to 20 meters, and for three different values of solar fluxes, that correspond to a sunny day (900W/m 2 ), an average day (600W/m 2 ) and a cloudy day (300W/m 2 ).

Results and discussions
Results of the tested sample. Aiming to obtain the experimental proof regarding the functionality of the device, a testing sample was built. The thickness of the sample was the same as in the description; the width of 0.56 m and height of 1 meter.
During the test period (from 28 th of September 2015 to 11 th of September 2016) daily experimental data was collected for input and output temperatures, and output air velocity. The solar air heater was protected from wind influence for its optimal functionality. The OMEGA HHF141 rotating vane anemometer was used to measure the output velocity value. The solar radiation intensity was recorded by the Yanishevskii pyrometer. Two temperature sensors (AS6200) were installed near the inlet and outlet of the device. All the data was collected automatically every 7.5 minutes throughout the testing period. Temperatures were also periodically measured at the bottom and top parts of hot the radiator place. The radiator temperature was on average 12 o C higher than the air 7 temperature in the nearest flow.
The average experimental data for each week is presented in Figure 4. It was observed that the output temperature depends on the input air temperature and, on average the solar air heater was heating up the air by 23. Initially, we built our version of the device in order to establish if there is an agreement of theoretical results with the experimental ones. In Figure 6 is presented the distribution of the temperature inside the air heater. We can observe that the temperature grows almost linearly along the metal board. After obtaining correspondence of the theoretical and experimental results we decided to run our simulation for different heights of the solar air heater. We propose that the industrial models of the device will have the same thickness, but a larger height and width adopted to building dimensions. In Figure 8 is presented the dependence of the output velocity and in Figure 9 of inlet-outlet temperature difference depending on the height of the heater. We can observe that the output velocity and temperature difference increase with the height of the device. The temperature difference increases slower than the velocity, because thermal dissipation becomes considerable.

Conclusions
The analysis of numerical simulations and experimental data lead us to conclude that this product can be used as an air heating system for residential and office buildings. It is also very suitable for green-houses heating, due to it working upon the principle of natural convection, to avoid airstreams and their negative effect on plant growth. For thermal comfort of the indoor environment the device can be used as a heating and ventilation system depending on the needs of the user. When outdoor temperatures are high and there is no need for the heating system, the solar air heater can be used to dry fruits, nuts and seeds.
Our solar air heater can replace the heating system during the autumn-spring period when the outdoor temperature is above −10 o C. The average thermal performance of the designed air heater is of η = 60.4% for the sun peak hours, and the device can significantly reduce heating costs and protect the environment. The heater can produce a flow of around 59 l/s of air that will be 23 o C hotter then outside temperature. Also, the solar air heater protects walls against temperature fluctuations, provides thermal insulation, and acoustic insulation, which lead to comfort increase.