Solar energy is the primary light and heat resource of the Earth. It can provide eternal energy to maintain the atmosphere temperature and germinate plants. With technological developments, solar energy can be utilized more and more efficiently and economically.
In a solar heating and cooling system, solar energy has the potential to meet a large proportion of the heating and cooling needs of buildings and industry. There are also numerous technologies for different heat source temperatures and specific demands. To ensure steady and long-term solar utilization, heat storage is also essential. In this post, an overview of the solar heating and cooling technologies will be given.
Solar energy is the energy source of solar heating and cooling systems. There are mainly two modern ways to collect solar energy. One is to directly adopt the thermal energy produced by solar radiation with use of a solar collector. The solar heat gained could be then transferred to solar heating or cooling applications; this kind of system is also called a solar thermal system. The other one is to transfer solar radiation into electrical power through photovoltaic (PV) material; this kind of system is also called the solar PV system.
When solar energy is integrated with the heating and cooling systems, there are many more options for thermal-driven systems than for electrical-driven systems. In this case, the solar thermal collectors are emphasized and thermal-driven systems have been extensively researched and developed. Because of the significant price reduction of solar photovoltaics in the last years, solar PV-powered systems are also becoming attractive.
There are different classifications of the solar collector. It can be classified into nonconcentrating types and concentrating types. It can also be classified into low-temperature collectors, medium-temperature collectors, and high-temperature collectors according to the working temperature. Low-, medium-, and high-temperature collectors work under 100°C, 100–200°C, and higher than 200°C, respectively.
In this post, solar collectors are classified into nontracking solar collectors and tracking solar collectors. A brief introduction of solar PV technology is also given.
Nontracking solar collectors
This type of solar collector mainly includes the flat-plate collector (FPC), the evacuated-tube collector (ETC), and the compound parabolic concentrator (CPC). They usually work as low- and medium-temperature collectors that are suitable for space-heating and space-cooling. Water, air, or oil can be used as a thermal transport medium.
FPCs: The FPCs usually contain the glazing, absorber plate, heat transfer component, and insulation layer. FPCs are typically used for space-heating or hot water supply. It has low working temperature, but it is simple, cost-effective, and has a long lifetime. It is also easily integrated in buildings.
ETCs: When the climate is not so warm or the working temperature is high, the FPC cannot work efficiently because of heat losses, and the ETCs can be used. In the ETC, the absorber surface with selective coating (absorptivity 95%, emissivity <5%) is placed in a double-layer tube with vacuum between two layers. The vacuum surrounding the absorber can greatly reduce the convection and conduction heat losses. In this case, the efficiency can be increased.
Compound Parabolic Concentrator
CPCs: To increase the solar collector efficiency, concentrating collectors such as CPCs can be used. The CPC is a nonimaging concentrator with a low concentration ratio. The CPC uses a compound parabolic reflective surface to reflect and concentrate the solar radiation to the focal line. A tubular absorber is used as a receiver. In some newly developed CPC collectors, a compound parabolic surface and receiver are integrated in the evacuated tube to avoid heat losses and increase the efficiency.
Tracking solar collectors
This type of solar collector mainly includes the single-axis tracking collectors and two-axes tracking collectors. Single-axis tracking collectors include linear parabolic trough collectors (PTCs), linear Fresnel reflectors (LFRs), and cylindrical trough collector (CTCs). They have a two-dimensional concentrating effect. Two-axes tracking collectors include the parabolic dish collector and solar tower (heliostat field) collector. They have a three-dimensional concentrating effect. The tracking collectors usually work as medium- and high-temperature collectors. Water, oil, or molten salt can be used as working fluid.
Parabolic Trough Collectors
PTCs: The PTC uses a parabolic trough reflector to concentrate the solar radiation. The tubular receiver integrated in the evacuated tube is placed along the focal line of the reflector. The collector needs to track the Sun along a single axis to maximize its efficiency. A higher concentration ratio than that of the CPC can be obtained. PTCs can effectively produce heat at temperatures between 50°C and 400°C. It can be used for solar thermal power generation, solar thermal energy for industry uses, and as the heat source for efficient solar cooling.
Linear Fresnel Reflectors
LFRs: The LFR uses several arrays of flat mirrors to reflect and concentrate the solar radiation together. Compared with PTCs, the LFR is cheaper and takes up less space. The mirror arrays are usually placed on the ground. This makes the installation easier than PTCs, especially in a large system. However, shading and blocking problems can possibly reduce its efficiency. Compact LFR technology can improve this now that it is well accepted for industry heating and solar cooling.
Parabolic dish: The parabolic dish utilizes the reflective dish to concentrate the solar radiation to one point. In this case the concentration ratio of a parabolic dish is higher than the PTC and LFR. Higher efficiency or higher working temperature can be obtained. The absorber of a parabolic dish collector is placed at the focal point. As three-dimensional concentrating is adopted, two-axes tracking is needed. Parabolic dishes have been used with power stirling engines to generate electricity.
Solar tower: The solar tower utilizes the heliostats to concentrate the solar radiation to the receiver on a tower. The heliostats are tracking mirrors spread around the tower. In this case the solar tower is also called the heliostat field or central receiver collector. Because the heliostats are individual components installed on the ground, the total reflective area and the concentration ratio can be large, which increases the system power and working efficiency. Solar tower systems have been considered as an efficient system to generate electricity from solar thermal power.The concentrating types, tracking modes, working temperatures, and efficiencies of the mentioned collectors are given in Table 1.1. The efficiencies of solar thermal collectors are closely related to the working temperature and ambient temperature. In this case the efficiencies are not included.
Solar thermal collectors
Indicative temperature (°C)
- CPC, Compound parabolic concentrator;
- PTC, parabolic trough collector;
- LFR, linear Fresnel reflector.
When solar photovoltaics are used for a heating and cooling system, a conventional vapor compression system can be adopted. In a solar PV system the solar radiation can be converted into direct current electricity through the PV effect of the semiconducting materials. Solar cells could be classified as silicon cells, thin film cells, emerging solar cells, and multijunction solar cells, among which silicon and film solar cells are available on the market.
Silicon cells: Silicon-based material is the most maturely developed and commercialized PV material. It is also called “first-generation” technology. Silicon-based materials account for the biggest market share for PV products. Multicrystalline silicon and monocrystalline silicon are the most commonly used materials on the market.
Thin film cells
Thin film cells: A thin film cell is made by depositing one or more thin layers of thin film PV material on a substrate. Its thickness varies from nanometers to tens of micrometers, which is easy for building integration. It is also called “second-generation” technology. Commercialized thin film solar cells typically use cadmium telluride, copper indium gallium selenide, and amorphous thin film silicon (a-Si). In 2014 thin film cells accounted for approximately 9% of worldwide deployment whereas the remainder comprised crystalline silicon cells.
Emerging solar cells
Emerging solar cells: The emerging solar cells can also be called the “third-generation” solar cells. These solar cells have the potential to overcome the Shockley–Queisser limit for single bandgap solar cells. They include the dye-sensitized cells and organic cells. Other available technologies include the copper zinc tin sulfide cell, perovskite cell, polymer cell, and quantum dot cell.
Multijunction cells: Traditional cells have only one p–n junction, and there is a theoretical efficiency limit. Multijunction solar cells have multiple p–n junctions made of different semiconductor materials. A theoretical efficiency up to 86.8% can be reached by infinite p–n junctions. The multijunction cells vary from the junction number and material. These include the InGaP/GaAs/InGaAs cell, amorphous silicon/hydrogen alloy (a-Si)/nanocrystalline or microcrystalline silicon (nc-Si)/nc-Si thin film cell, a-Si/nc-Si thin film cell, and so on.
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