A review of latest developments, progress, and applications of heat pipe solar collectors

Among all the available solutions to the current high energy demand and consequent economic and environmental problems, solar energy, without any doubt, is one of the most promising and widespread solutions. However, conventional solar systems face some intractable challenges affecting their technical performance and economic feasibility. To overcome these challenges, increasing attention has been drawn towards the utilization of heat pipes, as an efficient heat transfer technology, in conventional solar systems. To the authors’ knowledge, despite many valuable studies on heat pipe solar collectors (mainly during the last decade), a comprehensive review which surveys and summarizes those studies and identifies the research gaps in this field has not been published to date. This review paper provides an overview of the recent studies on heat pipe solar collectors (HPSCs), their utilization in different domestic, industrial, and innovative applications, challenges, and future research potentials. The concept and principles of HPSCs are first introduced and a review of the previous studies to improve both energy efficiency and cost effectiveness of these collectors is presented. Moreover, a concise section is dedicated to mathematical modeling to demonstrate suitable methods for simulating the performance of HPSCs. Also, the latest applications of HPSCs in water heating, desalination, space heating, and electricity generation systems are reviewed, and finally, some recommendations for future research directions, regarding both development and new applications, are made

Analysis of Thermoelectric Coolers as Energy Harvesters for Low Power Embedded Applications

The growing popularity of solid state thermoelectric devices in cooling applications has sparked an increasing diversity of thermoelectric coolers (TECs) on the market, commonly known as “Peltier modules”. They can also be used as generators, converting a temperature difference into electric power, and opportunities are plentiful to make use of these devices as thermoelectric generators (TEGs) to supply energy to low power, autonomous embedded electronic applications. Their adoption as energy harvesters in this new domain of usage is obstructed by the complex thermoelectric models commonly associated with TEGs. Low cost TECs for the consumer market lack the required parameters to use the models because they are not intended for this mode of operation, thereby urging an alternative method to obtain electric power estimations in specific operating conditions. The design of the test setup implemented in this paper is specifically targeted at benchmarking commercial, off-the-shelf TECs for use as energy harvesters in domestic environments: applications with limited temperature differences and space available. The usefulness is demonstrated by testing and comparing single and multi stage TECs with different sizes. The effect of a boost converter stage on the thermoelectric end-to-end efficiency is also discussed

A review of solar photovoltaic systems cooling technologies

Published ArticleCooling the operating surface is a key operational factor to take into consideration to achieve higher efficiency when operating solar photovoltaic systems. Proper cooling can improve the electrical efficiency, and decrease the rate of cell degradation with time, resulting in maximisation of the life span of photovoltaic modules. The excessive heat removed by the cooling system can be used in domestic, commercial or industrial applications. This paper presents a review of various methods that can be used to minimize the negative impacts of the increased temperature while making an attempt to enhance the efficiency of photovoltaic solar panels operating beyond the recommended temperature of the Standard Test Conditions (STC). Different cooling technologies are reviewed, namely Floating tracking concentrating cooling system (FTCC); Hybrid solar Photovoltaic/ Thermal system cooled by water spraying; Hybrid solar Photovoltaic/ Thermoelectric PV/TE system cooled by heat sink; Hybrid solar Photovoltaic/Thermal (PV/T) cooled by forced water circulation; Improving the performance of solar panels through the use of phase-change materials; Solar panel with water immersion cooling technique; Solar PV panel cooled by transparent coating (photonic crystal cooling); Hybrid solar Photovoltaic/Thermal system cooled by forced air circulation, and Solar panel with Thermoelectric cooling. Several research papers are reviewed and classified based on their focus, contribution and the type of technology used to achieve the cooling of photovoltaic panels. The discussion of the results has been done based on the advantages, disadvantages, area of application as well as techno-economic character of each technology reviewed. The purpose of this review is to provide an understanding for each of the above-mentioned technologies to reduce the surface temperature of the PV module. The study will focus on the surface temperature reduction array bound by each of the cooling technologies. The performance of each cooling technology will also be highlighted. In addition to this study, this review will include a discussion comparing the performance of each cooling technology. The outcomes of this study are detailed in the conclusion section. This paper has revealed that any adequate technology selected to cool photovoltaic panels should be used to keep the operating surface temperature low and stable, be simple and reliable and, if possible, enable the use of extracted thermal heat to enhance the overall conversion efficiency. The presented detailed review can be used by engineers working on theory, design and/or application of photovoltaic systems

Improving PV Module Efficiency Through Cooling

The Solarbacks researched and designed a variety of cooling methods that could be used to improve the efficiency of photovoltaics. These cooling methods can be separated into two categories: active and passive methods. The active cooling method of hydraulic cooling and the passive cooling methods of heat sinks (fins), optical coatings, thermosyphons, phase change materials, and thermoelectric generators were all taken into consideration as potential cooling methods. Passive cooling methods were preferred because the use of electricity required for the cooling mechanism would reduce the net electricity and subsequent profit from the panels. Two variations of hydraulic cooling were researched: water spraying and the use of closed channels along the back of the panel. Both water spraying and closed channels along the back of the panel could effectively cool down photovoltaics, but the energy required to pump the necessary amount of water would exceed the additional power generated from cooling. Both variations would also require significant capital cost and would be difficult to scale up. Two passive methods – thermosyphons and phase change materials – were researched but not tested as a final design. Thermosyphons use heat from the panel to boil a working fluid, increased buoyancy moves the fluid upwards where excess heat is released into the environment, condensing the fluid back into a liquid. This starts the process over again. Thermosyphons have been proven to work effectively for concentrated photovoltaic systems; however, the layout of typical solar farms is not conducive for thermosyphons if they utilize a solar tracking system. Chosen phase change materials would have a melting point that is within the operating range of the heated solar panel, and would cool the panel through conductive heat transfer from the back of the panel to the phase change material. When put in thermal contact with the panel, the panel’s temperature would not exceed the melting temperature of the material until all of it had melted. This method was disregarded because once the material had melted, the panel would no longer be cooled. Additional passive methods were researched and tested. Ideal optical coatings reflect any solar irradiance that is not used by the panel to produce electricity, however, the coatings researched and tested produced minimal cooling. The coating Solarbacks tested was a thin sheet of mylar (saran wrap). The average cooling produced by the saran wrap was about 2.4oC. However, most of this cooling is thought to be a result of a thermosyphon effect because the saran wrap was elevated off the surface of the panel rather than being directly attached. This elevation likely induced forced convection with the outside air to cool the panel. Fins as a heat sink work by increasing the surface area that heat can be dissipated from. One of the biggest disadvantages to fins is that their efficacy is strongly dependent on ambient conditions. The fins tested by Solarbacks were 1” tall, spaced 1” from each other, and placed on a 1/8” aluminum sheet and attached to the photovoltaic panel using a thermal mastic. The approximate cost of materials per panel would be around $28 when materials are purchased in bulk for a 1/32” thickness extruded fin. Testing showed that fins could cool the panel 14oC during peak temperatures and increase power output by about 5.52Thermal electric generators (TEGs) use electrically dissimilar semiconductors to produce an electric current. When put in thermal contact with the back of the panel, the generator would use any excess heat to produce electricity. The heat TEGs use to produce electricity could help cool the panel to some degree, but their main contribution is the additional electricity they generate. This additional electricity would outweigh the losses due to heating and increase the profitability of each solar panel. If the back of a panel was covered with TEGs and a 20oC temperature difference was maintained for 8 hr. a day in New Mexico, the TEGs would produce an additional 0.778 kWh/day. The biggest disadvantage to using TEGs is the capital cost. Using typical TEG dimensions (40mm*40mm), 536 of them would need to be bought per panel with each TEG costing about$2.92. Larger TEGs could be produced to fit to back of each panel and could reduce this capital cost significantly. Overall, TEGs with fins provides the greatest amount of panel cooling and additional power production. There is an average of a 12.1°C temperature difference along a panel with this solution installed. Using manufacturer data, an estimated 135W can be produced from the TEGs at a 20°C temperature differential along the TEGs. However, when payout for this method is considered, it would take nearly 31 years. Purchasing additional panels that produce the same amount of power as the TEGs would have a payout period of less than 6 years. TEGs with fins at their current cost is not an economic alternative to purchasing more panels despite its cooling and power production capabilities

Improving PV Module Efficiency Through Cooling

The Solarbacks researched and designed a variety of cooling methods that could be used to improve the efficiency of photovoltaics. These cooling methods can be separated into two categories: active and passive methods. The active cooling method of hydraulic cooling and the passive cooling methods of heat sinks (fins), optical coatings, thermosyphons, phase change materials, and thermoelectric generators were all taken into consideration as potential cooling methods. Passive cooling methods were preferred because the use of electricity required for the cooling mechanism would reduce the net electricity and subsequent profit from the panels. Two variations of hydraulic cooling were researched: water spraying and the use of closed channels along the back of the panel. Both water spraying and closed channels along the back of the panel could effectively cool down photovoltaics, but the energy required to pump the necessary amount of water would exceed the additional power generated from cooling. Both variations would also require significant capital cost and would be difficult to scale up. Two passive methods – thermosyphons and phase change materials – were researched but not tested as a final design. Thermosyphons use heat from the panel to boil a working fluid, increased buoyancy moves the fluid upwards where excess heat is released into the environment, condensing the fluid back into a liquid. This starts the process over again. Thermosyphons have been proven to work effectively for concentrated photovoltaic systems; however, the layout of typical solar farms is not conducive for thermosyphons if they utilize a solar tracking system. Chosen phase change materials would have a melting point that is within the operating range of the heated solar panel and would cool the panel through conductive heat transfer from the back of the panel to the phase change material. When put in thermal contact with the panel, the panel’s temperature would not exceed the melting temperature of the material until all of it had melted. This method was disregarded because once the material had melted, the panel would no longer be cooled. Additional passive methods were researched and tested. Ideal optical coatings reflect any solar irradiance that is not used by the panel to produce electricity, however, the coatings researched and tested produced minimal cooling. The coating Solarbacks tested was a thin sheet of mylar (saran wrap). The average cooling produced by the saran wrap was about 2.4oC. However, most of this cooling is thought to be a result of a thermosyphon effect because the saran wrap was elevated off the surface of the panel rather than being directly attached. This elevation likely induced forced convection with the outside air to cool the panel. Fins as a heat sink work by increasing the surface area that heat can be dissipated from. One of the biggest disadvantages to fins is that their efficacy is strongly dependent on ambient conditions. The fins tested by Solarbacks were 1” tall, spaced 1” from each other, and placed on a 1/8” aluminum sheet and attached to the photovoltaic panel using a thermal mastic. The approximate cost of materials per panel would be around $28 when materials are purchased in bulk for a 1/32” thickness extruded fin. Testing showed that fins could cool the panel 14oC during peak temperatures and increase power output by about 5.52Thermal electric generators (TEGs) use electrically dissimilar semiconductors to produce an electric current. When put in thermal contact with the back of the panel, the generator would use any excess heat to produce electricity. The heat TEGs use to produce electricity could help cool the panel to some degree, but their main contribution is the additional electricity they generate. This additional electricity would outweigh the losses due to heating and increase the profitability of each solar panel. If the back of a panel was covered with TEGs and a 20oC temperature difference was maintains for 8 hr. a day in New Mexico, the TEGs would produce an additional 0.778 kWh/day. The biggest disadvantage to using TEGs is the capital cost. Using typical TEG dimensions (40mm*40mm), 536 of them would need to be bought per panel with each TEG costing about$2.92. Larger TEGs could be produced to fit to back of each panel and could reduce this capital cost significantly. Overall, TEGs with fins provides the greatest amount of panel cooling and additional power production. There is an average of a 12.1°C temperature difference along a panel with this solution installed. Using manufacturer data, an estimated 135W can be produced from the TEGs at a 20°C temperature differential along the TEGs. However, when payout for this method is considered, it would take nearly 31 years. Purchasing additional panels that produce the same amount of power as the TEGs would have a payout period of less than 6 years. TEGs with fins at their current cost is not an economic alternative to purchasing more panels despite its cooling and power production capabilities

Improving PV Module Efficiency Through Cooling

The Solarbacks researched and designed a variety of cooling methods that could be used to improve the efficiency of photovoltaics. These cooling methods can be separated into two categories: active and passive methods. The active cooling method of hydraulic cooling and the passive cooling methods of heat sinks (fins), optical coatings, thermosyphons, phase change materials, and thermoelectric generators were all taken into consideration as potential cooling methods. Passive cooling methods were preferred because the use of electricity required for the cooling mechanism would reduce the net electricity and subsequent profit from the panels. Two variations of hydraulic cooling were researched: water spraying and the use of closed channels along the back of the panel. Both water spraying and closed channels along the back of the panel could effectively cool down photovoltaics, but the energy required to pump the necessary amount of water would exceed the additional power generated from cooling. Both variations would also require significant capital cost and would be difficult to scale up. Two passive methods – thermosyphons and phase change materials – were researched but not tested as a final design. Thermosyphons use heat from the panel to boil a working fluid, increased buoyancy moves the fluid upwards where excess heat is released into the environment, condensing the fluid back into a liquid. This starts the process over again. Thermosyphons have been proven to work effectively for concentrated photovoltaic systems; however, the layout of typical solar farms is not conducive for thermosyphons if they utilize a solar tracking system. Chosen phase change materials would have a melting point that is within the operating range of the heated solar panel, and would cool the panel through conductive heat transfer from the back of the panel to the phase change material. When put in thermal contact with the panel, the panel’s temperature would not exceed the melting temperature of the material until all of it had melted. This method was disregarded because once the material had melted, the panel would no longer be cooled. Additional passive methods were researched and tested. Ideal optical coatings reflect any solar irradiance that is not used by the panel to produce electricity, however, the coatings researched and tested produced minimal cooling. The coating Solarbacks tested was a thin sheet of mylar (saran wrap). The average cooling produced by the saran wrap was about 2.4oC. However, most of this cooling is thought to be a result of a thermosyphon effect because the saran wrap was elevated off the surface of the panel rather than being directly attached. This elevation likely induced forced convection with the outside air to cool the panel. Fins as a heat sink work by increasing the surface area that heat can be dissipated from. One of the biggest disadvantages to fins is that their efficacy is strongly dependent on ambient conditions. The fins tested by Solarbacks were 1” tall, spaced 1” from each other, and placed on a 1/8” aluminum sheet and attached to the photovoltaic panel using a thermal mastic. The approximate cost of materials per panel would be around $28 when materials are purchased in bulk for a 1/32” thickness extruded fin. Testing showed that fins could cool the panel 14oC during peak temperatures and increase power output by about 5.52Thermal electric generators (TEGs) use electrically dissimilar semiconductors to produce an electric current. When put in thermal contact with the back of the panel, the generator would use any excess heat to produce electricity. The heat TEGs use to produce electricity could help cool the panel to some degree, but their main contribution is the additional electricity they generate. This additional electricity would outweigh the losses due to heating and increase the profitability of each solar panel. If the back of a panel was covered with TEGs and a 20oC temperature difference was maintains for 5.5 hr. a day in New Mexico, the TEGs would produce an additional 0.778 kWh/day. The biggest disadvantage to using TEGs is the capital cost. Using typical TEG dimensions (40mm*40mm), 536 of them would need to be bought per panel with each TEG costing about$2.92. Larger TEGs could be produced to fit to back of each panel and could reduce this capital cost significantly. Overall, TEGs with fins provides the greatest amount of panel cooling and additional power production. There is an average of a 12.1°C temperature difference along a panel with this solution installed. Using manufacturer data, an estimated 135W can be produced from the TEGs at a 20°C temperature differential along the TEGs. However, when payout for this method is considered, it would take nearly 45 years. Purchasing additional panels that produce the same amount of power as the TEGs would have a payout period of less than 2 years. TEGs with fins at their current cost is not an economic alternative to purchasing more panels despite its cooling and power production capabilities

Improving PV Module Efficiency Through Cooling

The Solarbacks researched and designed a variety of cooling methods that could be used to improve the efficiency of photovoltaics. These cooling methods can be separated into two categories: active and passive methods. The active cooling method of hydraulic cooling and the passive cooling methods of heat sinks (fins), optical coatings, thermosyphons, phase change materials, and thermoelectric generators were all taken into consideration as potential cooling methods. Passive cooling methods were preferred because the use of electricity required for the cooling mechanism would reduce the net electricity and subsequent profit from the panels. Two variations of hydraulic cooling were researched: water spraying and the use of closed channels along the back of the panel. Both water spraying and closed channels along the back of the panel could effectively cool down photovoltaics, but the energy required to pump the necessary amount of water would exceed the additional power generated from cooling. Both variations would also require significant capital cost and would be difficult to scale up. Two passive methods – thermosyphons and phase change materials – were researched but not tested as a final design. Thermosyphons use heat from the panel to boil a working fluid, increased buoyancy moves the fluid upwards where excess heat is released into the environment, condensing the fluid back into a liquid. This starts the process over again. Thermosyphons have been proven to work effectively for concentrated photovoltaic systems; however, the layout of typical solar farms is not conducive for thermosyphons if they utilize a solar tracking system. Chosen phase change materials would have a melting point that is within the operating range of the heated solar panel, and would cool the panel through conductive heat transfer from the back of the panel to the phase change material. When put in thermal contact with the panel, the panel’s temperature would not exceed the melting temperature of the material until all of it had melted. This method was disregarded because once the material had melted, the panel would no longer be cooled. Additional passive methods were researched and tested. Ideal optical coatings reflect any solar irradiance that is not used by the panel to produce electricity, however, the coatings researched and tested produced minimal cooling. The coating Solarbacks tested was a thin sheet of mylar (saran wrap). The average cooling produced by the saran wrap was about 2.4oC. However, most of this cooling is thought to be a result of a thermosyphon effect because the saran wrap was elevated off the surface of the panel rather than being directly attached. This elevation likely induced forced convection with the outside air to cool the panel. Fins as a heat sink work by increasing the surface area that heat can be dissipated from. One of the biggest disadvantages to fins is that their efficacy is strongly dependent on ambient conditions. The fins tested by Solarbacks were 1” tall, spaced 1” from each other, and placed on a 1/8” aluminum sheet and attached to the photovoltaic panel using a thermal mastic. The approximate cost of materials per panel would be around $28 when materials are purchased in bulk for a 1/32” thickness extruded fin. Testing showed that fins could cool the panel 14oC during peak temperatures and increase power output by about 5.52Thermal electric generators (TEGs) use electrically dissimilar semiconductors to produce an electric current. When put in thermal contact with the back of the panel, the generator would use any excess heat to produce electricity. The heat TEGs use to produce electricity could help cool the panel to some degree, but their main contribution is the additional electricity they generate. This additional electricity would outweigh the losses due to heating and increase the profitability of each solar panel. If the back of a panel was covered with TEGs and a 20oC temperature difference was maintained for 8 hr. a day in New Mexico, the TEGs would produce an additional 0.778 kWh/day. The biggest disadvantage to using TEGs is the capital cost. Using typical TEG dimensions (40mm*40mm), 536 of them would need to be bought per panel with each TEG costing about$2.92. Larger TEGs could be produced to fit to back of each panel and could reduce this capital cost significantly. Overall, TEGs with fins provides the greatest amount of panel cooling and additional power production. There is an average of a 12.1°C temperature difference along a panel with this solution installed. Using manufacturer data, an estimated 135W can be produced from the TEGs at a 20°C temperature differential along the TEGs. However, when payout for this method is considered, it would take nearly 31 years. Purchasing additional panels that produce the same amount of power as the TEGs would have a payout period of less than 6 years. TEGs with fins at their current cost is not an economic alternative to purchasing more panels despite its cooling and power production capabilities

Space resources. Volume 2: Energy, power, and transport

This volume of the Space Resources report covers a number of technical and policy issues concerning the energy and power to carry out advanced space missions and the means of transportation to get to the sites of those missions. Discussed in the first half of this volume are the technologies which might be used to provide power and a variety of ways to convert power from one form to another, store it, move it wherever it is needed, and use it. In the second half of this volume, various kinds of transportation, including both interplanetary and surface systems, are discussed

Space power systems technology enablement study

The power system technologies which enable or enhance future space missions requiring a few kilowatts or less and using the space shuttle were assessed. The advances in space power systems necessary for supporting the capabilities of the space transportation system were systematically determined and benefit/cost/risk analyses were used to identify high payoff technologies and technological priorities. The missions that are enhanced by each development are discussed

Improving PV Module Efficiency Through Cooling

The Solarbacks researched and designed a variety of cooling methods that could be used to improve the efficiency of photovoltaics. These cooling methods can be separated into two categories: active and passive methods. The active cooling method of hydraulic cooling and the passive cooling methods of heat sinks (fins), optical coatings, thermosyphons, phase change materials, and thermoelectric generators were all taken into consideration as potential cooling methods. Passive cooling methods were preferred because the use of electricity required for the cooling mechanism would reduce the net electricity and subsequent profit from the panels. Two variations of hydraulic cooling were researched: water spraying and the use of closed channels along the back of the panel. Both water spraying and closed channels along the back of the panel could effectively cool down photovoltaics, but the energy required to pump the necessary amount of water would exceed the additional power generated from cooling. Both variations would also require significant capital cost and would be difficult to scale up. Two passive methods – thermosyphons and phase change materials – were researched but not tested as a final design. Thermosyphons use heat from the panel to boil a working fluid, increased buoyancy moves the fluid upwards where excess heat is released into the environment, condensing the fluid back into a liquid. This starts the process over again. Thermosyphons have been proven to work effectively for concentrated photovoltaic systems; however, the layout of typical solar farms is not conducive for thermosyphons if they utilize a solar tracking system. Chosen phase change materials would have a melting point that is within the operating range of the heated solar panel, and would cool the panel through conductive heat transfer from the back of the panel to the phase change material. When put in thermal contact with the panel, the panel’s temperature would not exceed the melting temperature of the material until all of it had melted. This method was disregarded because once the material had melted, the panel would no longer be cooled. Additional passive methods were researched and tested. Ideal optical coatings reflect any solar irradiance that is not used by the panel to produce electricity, however, the coatings researched and tested produced minimal cooling. The coating Solarbacks tested was a thin sheet of mylar (saran wrap). The average cooling produced by the saran wrap was about 2.4oC. However, most of this cooling is thought to be a result of a thermosyphon effect because the saran wrap was elevated off the surface of the panel rather than being directly attached. This elevation likely induced forced convection with the outside air to cool the panel. Fins as a heat sink work by increasing the surface area that heat can be dissipated from. One of the biggest disadvantages to fins is that their efficacy is strongly dependent on ambient conditions. The fins tested by Solarbacks were 1” tall, spaced 1” from each other, and placed on a 1/8” aluminum sheet and attached to the photovoltaic panel using a thermal mastic. The approximate cost of materials per panel would be around $28 when materials are purchased in bulk for a 1/32” thickness extruded fin. Testing showed that fins could cool the panel 14oC during peak temperatures and increase power output by about 5.52Thermal electric generators (TEGs) use electrically dissimilar semiconductors to produce an electric current. When put in thermal contact with the back of the panel, the generator would use any excess heat to produce electricity. The heat TEGs use to produce electricity could help cool the panel to some degree, but their main contribution is the additional electricity they generate. This additional electricity would outweigh the losses due to heating and increase the profitability of each solar panel. If the back of a panel was covered with TEGs and a 20oC temperature difference was maintained for 8 hr. a day in New Mexico, the TEGs would produce an additional 0.778 kWh/day. The biggest disadvantage to using TEGs is the capital cost. Using typical TEG dimensions (40mm*40mm), 536 of them would need to be bought per panel with each TEG costing about$2.92. Larger TEGs could be produced to fit to back of each panel and could reduce this capital cost significantly. Overall, TEGs with fins provides the greatest amount of panel cooling and additional power production. There is an average of a 12.1°C temperature difference along a panel with this solution installed. Using manufacturer data, an estimated 135W can be produced from the TEGs at a 20°C temperature differential along the TEGs. However, when payout for this method is considered, it would take nearly 31 years. Purchasing additional panels that produce the same amount of power as the TEGs would have a payout period of less than 6 years. TEGs with fins at their current cost is not an economic alternative to purchasing more panels despite its cooling and power production capabilities