Performance Improvements by Introducing Solar Pump in Flat Plate Collector – Experimental Study

Burning of fossil fuels is the main cause to many environmental issues including global warming, acid rains and draught. The rising of global demand for energy, coupled with the declining supply of fossil fuels and the associated environmental concerns, has significantly driven the global concerns on renewable energy sources [1-3]. Solar energy can be considered as a one of the most abundantly available type of renewable energy sources in developing world [4, 5]. In addition, solar energy is clean renewable energy source [1, 6], it has variable applications, including direct use for water heating, power generation, and space heating and cooling [7, 8]. Another utilization method of solar energy is by converting the electromagnetic energy of sun's light into direct electric current by using of photovoltaic cells [7]. The geographical location of the kingdom of Saudi Arabia (KSA) is in the middle of Arabian Peninsula with a latitude line between 32^oN and 17^oN and longitude lines between 56^oN and 28^oN [9, 10]. The studies conducted on KSA renewable energy potential in KSA showed a high potential in solar energy [9-11], whereas the mean value for the annual solar radiation in KSA is 2200 kWh/m² [9]. In addition, the annual average daily Global Horizontal Irradiation (GHI) values in Saudi Arabia is varying between 5700 Wh/m² and 6700 Wh/m² while the annual mean value of daily Direct Normal Irradiation (DNI) is varying between 4400 Wh/m² and 7300 Wh/m² with the maximum values in the northwest part of the country [9]. Solar energy research in KSA dates back several years, with initial planned investigations into solar energy systems launched by the King Abdulaziz City for Science and Technology in 1977 [12]. Recently, numerous studies on using of flat plate collectors were conducted in KSA. Nejlaoui et al. [13] developed a six sigma optimization for flat plate collector with an uncertain designing parameters for material, geometer and environment conditions for Qassim area. The collected results showed that the flat plate collector design presents 31% sensitivity based on efficiency and 27% sensitivity when considering the total cost. Khedher [8] has investigated experimentally the effect of varying water flow rates on flat plate collector efficiency. The collected results confirmed a maximum water tank temperature of 82.5℃ which was achieved at water flow rate 2.5 L/min. Another investigation on the flat plate system performance under different water flow rates was performed by Al-Ajlan et al. [14]. In this work, the investigation was focused on an active water heating system that uses flat plate solar. The collected results were derived from a simulation study, and the simulated outcomes were compared with experimental results collected from a flat plate collector in Riyadh's climate. The collected results from the simulation study and experimental investigation was with in good agreement. The effect of inlet flow condition on the efficiency of the FPCs was studied by Alobaid et al. [15]. The collected results in this work were validated from literature. Flat plate solar collectors are a widely used type of solar thermal collector that transforms solar energy into heat. FPCs are simple in design, they consist of a flat, dark-colored surface that absorbs solar radiation. This heat is then passed on to a circulating fluid, typically water or antifreeze, which can be used for various applications [16]. Flat plate collectors (FPCs) are considered as one of the cheapest and most desirable equipment for water heating for domestic use. In passive (FPCs), water circulation can be accomplished naturally where water flows due to density differences and convection current [1]. Although the relatively high efficiencies associated with passive (FPCs), high temperatures are very difficult to be achieved with such systems due to poor water circulation associated with natural convection [1], other reasons limit the system to achieving high temperature for instance, flat plate collectors have a relatively simple design, which can lead to significant heat losses. These losses which caused by conduction, convection, and radiation, decrease the overall efficiency of the collector [17, 18]. Also design limitations, the design of flat plate collectors often involves a simple absorber plate and a glazing layer, these components may not be able to withstand extremely high temperatures without degrading or failing [7, 19, 20]. Achieving of high temperature is very important parameters in several solar heating applications such as water desalination and water heating for domestic use purposes. Different methodologies can be implemented to achieve higher temperatures in FPCs such as using of concentrating techniques [21, 22], using of Nano-fluids [23, 24] and using of active FPC systems [16, 25]. As compared with FPC active systems, using of concentrating techniques and Nano-fluids to improve the FPCs performance can be considered relatively expensive, where additional materials and space may require. Active FPCs systems use an external pump to circulate the water inside the system, which would affect in improve of the heat transfer coefficient, and hence an increase in water temperature is expected. On the other hand, additional power is needed to run the pump in active FPCs, which in turn would reduce the overall efficiency. The commonly available active FPCs are use electric pump to move the water within the collector which reduce the overall system's efficiency and increase the cost. Use of small size PV solar pump to circulate water in FPCs would give an opportunity to improve the performance of such systems without consuming an electric power.

The traditional FPC that use Thermosiphoning techniques of natural water circulation suffering of a number of issues such as the big size of the system, heat loss and relatively low achievable temperature. Research into more effective FPC systems with small sizes and higher efficiency is crucial. The integrated FPC with a solar-powered water circulation pump offers a sustainable solution for water heating and circulation, particularly in regions with abundant solar resources like KSA. However, improvements in design, materials, and manufacturing processes can significantly boost efficiency, longevity, and cost-effectiveness.

The integration of FPC with low cost small size solar would provide the system with a great improvement in the efficiency since the small size solar pump is not requires big collector area and provides a quite enough water circulation which will improve the heat transfer coefficient significantly. In addition, the use of solar pump provides a sustainable solution for water circulation inside FPC system with an environmentally friendly source for water circulation. Moreover, Integrating the FPC with other renewable energy technologies, such as photovoltaic (PV) modules, can create hybrid systems that provide both heat and electricity. Combined FPC with solar-powered water pumps provides a sustainable solution for water heating in regions with abundant of solar energy like KSA. Continued research and development in these areas are crucial for realizing the full potential of such technology.

Saudi Arabia known with its huge solar resources specially in northern and northwestern parts of the country which put the utilization of solar energy as an attractive option for different applications such as space cooling, water desalination and water heating. Despite the great efforts being made in the Kingdom of Saudi Arabia to benefit from the available renewable resources, especially solar energy, all these efforts are focused on electricity generation and there are no tangible efforts to use solar energy in simple direct thermal applications. The use of small size PV pumps to circulate the water in FPCs would improve the collected water temperatures to a higher level without high cost. Additionally, the small size PV pumps are using very small areas for PV collector which will not affect too much on the required area. The collected results from this work showed that the improved FPC could be used as a promising reliable solution for water heating for domestic uses in Qassim region and northern of Saudi Arabia especially in winter seasons where a considerable amount of electricity is consumed in water heating.

During conducting the experiments, there is a need to calculate different parameters related the performance of the fabricated system. These parameters include collector thermal efficiency, thermal useful load of the collector and collector losses.

3.1 Thermal useful load on the FPC

At the steady operation condition, the different heat losses in the flat plate collector can be represented as in overall heat loss coefficient. The amount of heat collected by the system for specific area (Ac) can be calculated based on Eq. (1) [1].

$Q_u=A_c\left[G_t(\tau \alpha)-U_L\left(T_p-T_a\right)\right]$     (1)

where,

$\mathrm{Q}_{\mathrm{u}}$=Themal useful load

$G_t$=Global solar irradiance at the collector plane.

Ac=Collector area (m²) including FPC area and solar pump collector area.

$\tau \alpha$=Absorber transmittance.

$U_L$=Solar collector overall heat loss coefficient ($\mathrm{W} / m^2$ - ℃).

$T_p$=Average temperature of the absorbing surface (℃).

$T_a$=Ambient temperature (℃).

3.2 Top loss coefficient

As in all thermal energy conversion systems which include heat transfer processes, heat losses to the surrounding by various means of heat transfer are not avoidable. The heat loss from the flat plate collector includes top loss, bottom and edge loss. Since the top part of the collector is exposed directly to atmospheric condition and cannot be insulated such as collector’s bottom the majority of the collector heat loss is due to the top loss which is calculated based on Eq. (2) [1].

$\begin{gathered}U_t=\frac{1}{\frac{N_g}{\frac{C}{T_p}\left(\frac{T_p-T_a}{N_g+f}\right)^e+\frac{1}{h_w}}}+\frac{\sigma\left(T_p^2+T_a^2\right)\left(T_p+T_a\right)}{\frac{1}{\epsilon_p+0.00591 N_g h_w}+\frac{2 N_g+f-1+0.13 \epsilon_p}{\epsilon_g}-N_g}\end{gathered}$     (2)

where,

$f=\left(1+0.089 h_w-0.1166 h_w \epsilon_p\right)\left(1+0.07866 N_g\right)$.

$C=520\left(1-0.000051 \beta^2\right)$. 

$e=0.43\left(1-\frac{100}{T_p}\right)$ .

$N_g=$ Number of glass covers.

$\sigma=$ Stefan - Boltzmann constant $=5.67 * 10^{-8} \mathrm{~W} / \mathrm{m}^2 . \mathrm{K}^4$.

$\epsilon_{\mathrm{g}}=$ Emissivity of glass covers.

$\epsilon_{\mathrm{p}}=$ Absorber plate emittance.

$\mathrm{T}_{\mathrm{p}}=$ Average temperature of the absorbing surface $\left({ }^{\circ} \mathrm{C}\right)$.

$\mathrm{T}_{\mathrm{a}}=$ Ambient temperature $\left({ }^{\circ} \mathrm{C}\right)$.

$\mathrm{h}_{\mathrm{w}}=$ Wind heat transfer coefficient $\left(\mathrm{W} / \mathrm{m}^2{ }^{\circ} \mathrm{C}\right)$.

$\mathrm{L}=$ Half distance between two consecutive riser pipes (m).

$\mathrm{V}=$ Wind velocity $(\mathrm{m} / \mathrm{s})$.

3.3 Bottom heat loss coefficient

Collector bottom heat loss is mainly due to conduction. Since the bottom side is insulated, the temperature at the outside surface of the bottom which exposed to atmospheric condition is expected to be low and thus heat transfer due to the radiation can be neglected. Collector bottom heat transfer coefficient can be calculated based on Eq. (3) [1].

$U_b=\frac{1}{\left(\frac{t_b}{k_b}+\frac{1}{h_{c e-a}}\right)}$     (3)

where,

$t_b$=Thickness of back insulation (m).

$k_b$=Conductivity of back insulation (W/m ℃).

$\mathrm{h}_{\mathrm{ce}-\mathrm{a}}$=Convection heat loss coefficient from back to ambient (W/$m^2$ -K).

3.4 Heat loss coefficient form the collector edges

As compared with top and bottom area, flat plate collector edges have smaller area which would affect in reducing of amount of heat loss. The heat transfer coefficient for the flat plate collector edges can be calculated based on Eq. (4) [1].

$U_e=\frac{1}{\left(\frac{t_e}{k_e}+\frac{1}{h_{c, e-a}}\right)}$     (4)

where,

$t_e$=Thickness of edge insulation (m).

$k_e$=Conductivity of edge insulation (W/m ℃).

$\mathrm{h}_{\mathrm{c}, \mathrm{e}-\mathrm{a}}$ =Convection heat loss coefficient from edge to ambient ($\mathrm{W} / m^2$ ℃).

3.5 Solar collector overall heat loss coefficient

Collector's overall heat loss coefficient was calculated based on Eq. (5).

$U_L=U_t+U_b+U_e$    (5)

3.6 Thermal efficiency

Collector's thermal efficiecny was calculated based on Eq. (6).

$\eta=\frac{Q_u}{A_c G_t}$    (6)

where, $\eta=collecto's \ thermal \ efficiency$.

3.7 Heat removal factor

In all flat plate collectors, heat removal factor is very important parameter since it represents to which extent the available energy es transferred to the working fluid. Heat removal factor can be defined as the ratio of the amount of useful energy gained and the amount that would result if the absorbing plate surface of the collector had been at the same local fluid temperature. The collector heat removal factor was calculated based on Eq. (7) [1].

$F_R=\eta /\left[(\tau \alpha)-U_L \frac{(T p-T a)}{G_t}\right]$     (7)

where,

$G_t$=Global solar irradiance at the collector plane.

$F_R$=Heat removal factor.

$\tau \alpha$=Absorber transmittance.

$U_L=$=Solar collector overall heat loss coefficient (W/ $m^2$ - ℃).

$T_p$=Average temperature of the absorbing surface (℃).

3.8 Reckoning of time

Since there is a variation between the local time at specific location and the real position of the sun in the sky, apparent solar time is a suitable way express the day time with the real position of the sun. For all tested hour modes, the local noon time for Unaizah city was found to be at 12:12 PM which was calculated based on the apparent solar time based on the Eq. (8) [1].

$A S T=L S T+E T \pm 4(S L-L L)$     (8)

where,

$E T=9.87 \ sin (2 B)-7.53 \ cos (B)-1.5 \ sin (B)$     (9)

where,

$B=(N-81) \frac{360}{364}$ .

N=Day Number (From table).

AST=Apparent solar time.

LST=Local standard time.

ET=Equation of time.

SL=Standard longitude.

LL=Local longitude.