New Design of Solar Photovoltaic and Thermal Hybrid System for Performance Improvement of Solar Photovoltaic
Solar photovoltaic (PV) and solar thermal systems are most widely used renewable energy technologies. Theoretical study indicates that the energy conversion efficiency of solar photovoltaic gets reduced about 0.3% when its temperature increases by 1°C. In this regard, solar PV and thermal (PVT) hybrid systems could be a solution to draw extra heat from the solar PV panel to improve its performance by reducing its temperature. Here, we have designed a new type of heat exchanger for solar PV and thermal (PVT) hybrid systems and have studied the performance of the system. The PVT system has been investigated in comparison with an identical solar PV panel at outdoor condition at Dhaka, Bangladesh. The experiments show that the average improvement of open circuit voltage (Voc) is 0.97 V and the highest improvement of Voc is 1.3 V. In addition, the overall improvement of output power of solar PV panel is 2.5 W.
Introduction
Solar photovoltaic (PV) and solar thermal systems are among the most widely used renewable energy technologies. However, the solar PV and thermal (PVT) hybrid system is not popular to that extent. Solar PV systems are available in different sizes from single solar panel system to large-scale solar PV power plants. And solar thermal systems are mostly used for household and commercial purposes. The operation of solar photovoltaic is highly dependent on weather conditions like temperature, precipitation, air condition, and most importantly solar irradiance. Like all other semiconductor devices, solar PV cells are sensitive to temperature. With the increase in temperature, the Band gap energy of semiconductor material gets reduced, so the current increases slightly but the voltage decreases significantly. Therefore, the energy conversion efficiency of silicon solar cell reduces about 0.3% when its temperature increases by 1°C [1, 2]. The working temperature of solar cell could be as much as 50°C above the ambient temperature and consequences of high working temperature are, drop in cell efficiency and permanent structural damage of the solar panel if the thermal stress remains for a longer period [3].
Equation (1) shows how temperature is related to current and voltage of a solar cell, where is the dark current, is the photo current,
is the charge of electron,
is Boltzmann’s constant, and
is the temperature. The simulation result (Figure 1) based on the current Equation (1) of silicon solar cell shows that the voltage of the cell reduces noticeably when its temperature increases. However, the increment of cell current due to temperature raise is negligible. Therefore, the output power of solar cell decreases with the increase in temperature.
The impact of temperature on current and voltage of a solar cell. The reduction in voltage is higher than the increase in current; therefore, the output power of solar cell decreases with increase in temperature.
When thermal energy system is integrated with the solar photovoltaic system, it is called the photovoltaic and thermal (PVT) hybrid system [4]. Since, the hybrid system utilizes same area for the production of electricity and heat energy, it increases the overall efficiency of the system in terms of energy generation from per unit area [5]. The solar PVT system can be used for crop drying and air heating, and it can also be integrated with building facade, known as the building integrated PVT system (BI/PVT) [6]. The theoretical calculation says that the overall energy conversion efficiency of a hybrid system could be 60–80% [7]. However, the first priority of most of the hybrid systems is to bring down the temperature of solar PV, so that its electrical performance can be improved [8]. Different types of heat exchanger have been integrated with solar PV panel for improving its efficiency by reducing its temperature, for example, the use of fins, hexagonal honeycomb heat exchanger, and V-grooved absorber [9]. Three different designs like V-groove, honeycomb, and stainless steel wool have been installed horizontally into the channel located at the back side of a solar PV panel to improve performance of the PV panel [10]. In another case, 12 units of rectangular tunnel have been installed parallel at the back side of a solar PV panel as a heat exchanger to improve the efficiency of the system [1, 11]. It has been reported that four different hybrid systems like (i) PV with water flow, (ii) PV with water flow and glazing, (iii) PV with air circulation, and (iv) PV with air circulation and glazing have been experimented based on commercial PV panel at outdoor conditions. The result shows that PV cooling can increase the electrical efficiency of the PV panel. The efficiency can improve further by using a booster diffuse reflector [12]. However, the hybrid system with solar PV panel underneath of a glazing system has its own disadvantages. The electrical efficiency of the panel reduces due to the absorption and reflection of sunlight by the glazing system [13]. Recent study shows that the water-based PVT system can improve the power of the solar PV panel by 6% in average compared to the conventional PV panel [14]. Another study shows that the top surface of the solar PV panel cooling by water can improve the panel efficiency by almost 1.5% [15]. However, the water-based PVT system requires more energy to circulate the working fluid and the system is more complex. Different PVT systems have been tested under same environmental conditions using artificial neural network- (ANN-) based multilayer perceptron (MLP) system [16]. The study shows that the nanofluid/nanophase change material- (PCM-) based system enhances both the electrical and thermal efficiency. It also improves the voltage significantly. Among various PVT systems such as air, water, air/water, phase change material (PCM), and nanofluid, it is found that the air heater PVT system is promising for future preheating air applications [17]. The PVT system has the potential to integrate thermoelectric generator with the system to produce electricity from the thermal energy that is extracted from the solar panel [18, 19].
Here, we have proposed a hybrid system with a new type of heat exchanger for improving performance of the solar PV panel. As shown in Figure 2, the heat exchangers are arranged in such a way that they guide air to circulate in waveform through the channels located inside the solar thermal system. The upper side of heat exchangers is bended and is attached to the back side of the solar PV panel so that conduction heat transfer can happen from the PV panel to the heat exchanger. Performance of the PVT system has been investigated with respect to a solar PV panel with identical specification. Both the systems, solar PV and thermal (PVT system) and normal solar PV panel (normal system), have been examined simultaneously at outdoor conditions.
Schematic of solar thermal system. The solar PV panel is to set on the top of the solar thermal system.
Materials and Method
The solar PV panel attached to the heat exchanger and internal construction of the heat exchanger are shown in Figures 3(a) and 3(b), respectively. The encloser of the heat exchanger is made of corrosion resistive stainless-steel sheet, and exposed surfaces to air of the heat exchanger are insulated using the glass wool. The fins guide air circulation; the channels are made of aluminium. The upper side of fins is bended and tightly attached to the back surface of the solar PV panel, so that heat transfer from the PV panel to fins occurs by the conduction process. Equation (2) is the heat flow rate across materials by conduction.
where is the thermal conductivity of the material, is the heat transfer area, is the temperature difference across the materials, and is the thickness. So, the conduction process of heat increases with the increase in thermal conductivity of the material and increase in the heat transfer area. But the heat transfer decreases with the thickness of the transferring material. Thermal conductivity of aluminium is less than that of copper, but aluminium is cost effective, so it has been used as heat transferring material. Since heat transfer rate is inversely related to thickness of heat exchange material, therefore, thin aluminium sheet (1 mm of thickness) has been used as heat exchange material. The heat transferring area of the sheet has been kept as long as possible to increase the heat transfer.
(a)
(b)
(a) (b)
(a) The solar PV panel installed on the top of the heat exchanger. (b) Internal construction of the heat exchanger.
Performance of the PVT system has been evaluated with respect to an identical solar PV panel at outdoor environmental condition. Identical multicrystalline solar PV panels of 50 W and 12 V rating have been used in both PVT and normal systems [20]. Both the systems have been measured simultaneously at outdoor condition for several days. The experiments are carried out at Dhaka, Bangladesh. Digital multimeters have been used to measure the voltage and current of solar PV panels. The temperature of air at inlet and outlet of the PVT system has been measured by thermocouple. The experimental setup for performance evaluation is shown in Figure 4.
(a)
(b)
(a) (b)
Digital multimeter is connected to measure the voltage and current. (a) The PVT system and (b) the normal system.
Results and Discussion
Different parameters, for example, air temperature at inlet and outlet of the PVT system, voltage and current of the PVT system and the normal system have been measured to evaluate performance of the systems. The power output of the solar PV panels is calculated based the on the voltage and current readings. Temperature of air at intel and outlet of the PVT system, open circuit voltage, and short circuit current readings of the systems have been taken in every 10 min interval.
3.1. Temperature of Air at Intel and Outlet of PVT System
Except two readings, temperature of air at outlet of the PVT system is always higher than temperature of air at inlet of the system as shown in Figure 5. It indicates that the heat is effectively transferred from the PVT system to the air when the air is circulating through the channels of the heat exchanger. Average temperature difference of air between outlet and inlet is 2.6°C, and maximum difference is more than 4°C. The result shows that the heat exchanger works properly to transfer heat from the PVT system to the circulation air and it should help to improve the electrical output of the PVT system. The air flow rate is an important parameter to performance evaluation of the solar PV and thermal hybrid system. The air flow rate through the heat exchanger can be regulated by controlling the speed of the cooling fan. In our experiments, we have used natural air flow and have not measured the air flow rate.
Temperature of air at inlet and outlet of the PVT system. Average difference in temperature of inlet air and outlet air is 2.6°C, and maximum difference in temperature is more than 4°C.
3.2. Open Circuit Voltage and Short Circuit Current
The open circuit voltage (Voc) measurement of the PVT and the normal systems are shown in Figure 6. There is a significant improvement in Voc of the PVT system compared to the normal system. The Voc of the PVT system is always higher than the Voc of the normal system throughout the day. This result indicates that the heat exchanger is successfully transferring heat from the PVT system to the circulating air. The average improvement in Voc is 0.97 V, and the maximum improvement in Voc is 1.3 V. On the other hand, the reduction in short circuit current (Isc) of the PVT system compared to the normal PV system in not significant as shown in Figure 7. The average reduction in Isc of the PVT system is 0.04 A compared to the normal system and maximum reduction in Isc is 0.16 A. The air temperature readings at inlet and outlet of the PVT system and voltage and current readings of the PVT and normal systems support each other.
Hybrid Solar Panel, Two in One
A hybrid solar panel is a technological innovation that combines photovoltaic and thermal energy. This technology, still relatively unknown, is a “two-in-one” solution that is more efficient, economical and environmentally friendly than simple photovoltaic or thermal energy. Thus, this solar technology makes it possible to generate electricity and hot water simultaneously for the same building. The general idea of the hybrid solar panel is to be a combination of traditional photovoltaic technology and solar thermal collectors. However, Abora Solar and its aHTech technology has gone a step further in hybrid solar technology by innovating even more to offer the most efficient and effective solar panel in the world.
How does a hybrid solar panel work?
The hybrid solar panel has a very intelligent operation. In simple terms, the hybrid solar panel has high efficiency thermal collectors on the back of the panel and photovoltaic solar cells on the front. These convert solar energy into electricity and at the same time the thermal collectors recover the heat emitted by the sun through a heat transfer fluid or hot air collector.
Thus, it is possible to generate electricity and heat simultaneously thanks to its two clearly differentiated operating layers:
- The top layer consists of photovoltaic cells that produce electricity by capturing protons emitted by solar radiation.
- The bottom layer is equipped with a solar thermal collector that captures the heat emitted by the sun.
Combining the characteristics of photovoltaic and thermal panels, hybrid solar panels, known as PV/T, are based on the principle of solar cogeneration, which allows them:
- Generate electricity to light your home and power all the appliances installed in your home from natural energy.
- It produces heat to heat water, a swimming pool, ambient air or even to run your underfloor heating system.
What are the differences?
Let’s go back to the photovoltaic solar panel. In general, it is capable of capturing 20% in the form of photovoltaic energy. The remaining 80% of the energy is lost through reflection of the sun’s rays and heat loss.
The PVT, on the other hand, integrates a photovoltaic panel and a water circuit at the rear. In theory, it converts the thermal losses at the back of the panel (40%) into solar thermal energy. This implies an efficiency of 60%.
In practice, however, this is far from the case. In fact, heat loss to the rear is lost through the front. Only 5% is converted into solar thermal energy. Therefore, the total efficiency is only 25%.
Abora Solar’s hybrid solar panel is equipped with aHTech technology that reduces heat loss. The solar thermal energy is increased by up to 70%, as well as the photovoltaic energy. The efficiency of the panel with aHTech is then 89%. With aHTech technology, you will have the most efficient hybrid solar panel in the world, as our technology produces the same energy as 4 photovoltaic solar panels and takes advantage of 89% of the solar irradiation.
Benefits of hybrid solar panels with aHTech technology
- Higher generation per square metre of aHTech® technology.
- Higher performance of the technology
- Greater solar contribution of DHW per square metre, for the same collector surface area, twice the solar coverage of the hot water in the building is achieved.
- Greater economic savings
- Lower payback with aHTech® technology.
- Higher IRR.
- Higher cumulative cash flow
- Higher emission reduction, 50% more emissions avoided thanks to aHTech® technology.
- Lower energy price cost, 50% less with aHTech®.
Hybrid solar energy: Installation and price
You don’t need to live in the south to consider installing a hybrid solar panel. In fact, you can install hybrid solar panels and enjoy all the advantages, all you need is…. a roof! Installing hybrid solar panels involves attaching them to the roof, installing the wiring and electrical equipment and connecting them to the building’s hydraulics.
Every installation is different because every roof is different. But don’t worry, thanks to our MyHybridProject app you can get a quote for your installation in just a few clicks.
Hybrid Solar Panels: A Guide to PVT Systems
Hybrid solar panels combine the technology of #PV and thermal panels to produce both heat and electricity. Here’s what you need to know before considering them for your home.
Hybrid solar panels, or PVT #Solar panels, are a combination of solar photovoltaic panels and solar thermal panels in one module. A hybrid solar PVT module can therefore produce both electricity and heat simultaneously
While combining these systems may sound like a no-brainer, the technology does have limitations in comparison to separate PV and thermal #solarpanels.
This includes specialist installation with increased costs and a reliance on a primary heating system when using hybrid solar panels.
However, there are some instances where hybrid panels are the perfect choice for supplementing your home’s energy supply. We explore the technology in more detail.
How Do Hybrid Solar Panels Work?
Hybrid solar panels are effectively a solar PV panel that also has pipes that are built into the collector with a fluid circulating between them and a water cylinder.
As the sun shines on the panel the light is absorbed by the PV cells and the heat is absorbed by the solar thermal element.
- The fluid warms up and can be used as useful heat
- The fluid cools the PV cells which makes them more efficient.
Pros and Cons of Hybrid Solar Panels
Hybrid solar panels take up less space on a roof because the solar PV and the solar thermal panels are combined. This could be ideal on homes that have smaller roofs, such as three-storey properties.
However, solar PVT panels can be expensive. They are not a mainstream product yet so the installers and materials could be harder to source and therefore more costly. There are a few different solar PVT manufacturers but the products are very similar.
The combined nature of the way these panels work is a good concept and optimises the PV. If production increases then we may see economies of scale that means the panels could then be financially competitive and more widely specified.
Further, the thermal element of the PVT panel will also not get to the same high temperatures reached by standard solar thermal panels. This makes the heat more difficult to use as it requires additional heating.
This means that PVT panels are a complimentary system and you will still need a primary heating system.
Given the cost of the solar PVT panels and the relatively small number of installers, the general consensus is that if you have enough room on the roof then going for separate solar PV and solar thermal systems could be the best option.
The solar PV and the solar thermal panel systems can then be sized properly and the energy use optimized.
How Much Do Hybrid Solar Panels Cost?
The cost of solar PVT systems ranges depending on the manufacturer, the capacity of the system, components included and the installer. As there are not a lot of approved and experienced installers of these systems, the price could also be affected quite significantly by travel costs.
A 4kWp system could cost around £10,000 installed.
This compares to around £5,000. £8,000 each for PV and solar thermal panels.
How Much Energy do Hybrid Solar Panels Generate?
According to manufacturers, a solar PVT system can generate around 1500kWh of energy per kWp installed per year. That would be around 1000kWh of electricity and around 500Wh of heat.
The hybrid solar PVT panels can produce more heat than this but that could then be too hot for the PV cells. The crucial design details would be to make sure that you can use all the generated energy but also not overheat the PV cells.
Most manufacturers will have software that can give you a reasonable indication of your annual energy generation and you can then use this to evaluate the return on investment.
How is a Hybrid Solar Panel Installed?
Hybrid solar panels are installed in the same way that regular solar panels are.
Roof hooks are fixed to the roof trusses or rafters, depending on where the panels will sit in relation to the roof cladding, to which aluminum rails are then fixed horizontally. The panels are bolted to these rails.
Solar PVT panels will require the wires from the PV function to lead back to an inverter to turn it into usable energy, as well as pipes connecting to the home’s hot water storage for its solar thermal component.
As PVT systems are more specialist at present, the best way to find the right installer is to ask the manufacturer of your product for a list of approved installers.
Hybrid Solar Panels vs Other Solar Hybrid Technology
Don’t confuse hybrid solar panels with Hybrid Solar air systems also referred to as aerovoltaic. This is where ducts are built into the photovoltaic panel, through which air is drawn across the panel. This is delivered to the home to cool the PV panel but also preheat the fresh air entering the home.
Thermodynamic panels are also often confused with solar PVT. These technologies have collectors that are mounted on a roof or a wall and have refrigerants in them that absorb the sun’s energy.
They do not generate electricity and effectively a type of heat pump system.
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World has technical potential to host 47.6 PWh of photovoltaic-thermal panels
Scientists from the Central European University in Hungary have estimated the global technical potential of photovoltaic-thermal (PVT) energy production by using a high-resolution geospatial model.
Mean potential of solar thermal energy production on 1 square meter of available rooftop space
Image: Central European University, The Journal of Cleaner Production/ CC BY license
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Researchers from the Central European University in Hungary have estimated the global potential of photovoltaic-thermal (PVT) energy production on rooftops between 2022 and 2060.
Their model predicts that PVT systems have the technical potential to produce curently around 47.6 PWh, with the contribution on the PV side being around 29.5 PWh (62%), and that of thermal production 18.1 PWh (38%).
The study uses a high-resolution, geospatial energy supply model to estimate the useable building rooftop areas across eleven world regions and calculates the corresponding global and regional production that should be provided by rooftop PV-T collectors during the 39-year perios. The model builds on an existing model called BISE, which has three major types of inputs: meteorological, building-related and technological parameters. According to the authors, the strength of their algorithm is that “it requires only easily obtainable and reproducible input data for meteorological and technological measures.”
The scientists assumed the electric and thermal efficiency of the PV-T collectors to be of 21.6% and 63.3%, respectively, as well as a temperature coefficient of.0.35% C for the PV unit. They also assumed the PV panels to rely on monocrystalline cells and an antireflective tempered glass and the thermal modules to be based on a flat plate collector and a copper absorber. For both technologies, degradation rates and possible technology advancements were not considered.
The model estimated total rooftop area values to vary between 3 billion square meters in Europe and 70 billion square meters in centrally planned Asia, including China, in the first year of the simulation period. However, only one-third of the total rooftop area was considered suitable for the installation of PV-T collectors.
By 2060, the highest installable rooftop area is expected in centrally planned Asia, Latin America, and North America. The lowest area is projected for Sub Saharan Africa, at 4.75 billion billion square meters, and Europe, at 1.2 billion billion square meters.
Total PV-T energy production potential is expected to be highest in centrally planned Asia, North America, and Latin America, with 9.7 PWh, 6.0 PWh, and 4.5 PWh, respectively.
“The most restrained potentials, on the other hand, are expected in Europe (0.24 PWh) and Sub Saharan Africa (1.28 PWh) where either the geographical (climatic) or the utilization potential of solar collectors may be reduced,” the scientists said.
Between 2022 and 2060, Latin America, the Asia-Pacific region, and the Middle East and North Africa (MENA) region are expected to see the greatest increase in PV-T energy production potential.
“As the flexibility and the complexity of the BISE model allows and it could lead to even more robust estimations, our forthcoming studies will also prioritize simulations of solar potential undertaken at urban and neighborhood scale and supported by very high-resolution LIDAR-based rooftop information,” they said.
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Beatriz Santos
Beatriz Santos joined pv magazine in 2020. She wrote about new technology, RD, installations, markets, and policy on the global website.