4.6 Solar Thermal Collectors And Applications

Introduction

Solar thermal technologies use sunlight to produce heat rather than electricity. In this chapter the focus is on how different types of solar thermal collectors capture solar energy, how they move and store heat, and where they are used in practice. Concepts such as solar radiation and the general comparison between thermal and photovoltaic are covered elsewhere, so here the emphasis stays on the specific designs and applications of thermal collectors.

Basic Working Principle Of Solar Thermal Collectors

All solar thermal collectors follow the same chain of steps. Sunlight reaches a surface that is designed to absorb as much of the incoming radiation as possible. This surface, often called an absorber plate, heats up. A fluid, usually water, air, or a heat transfer liquid such as glycol, flows in contact with or near this absorber. As it passes through the collector, the fluid removes heat from the absorber and carries it to where it is needed, for example a hot water tank or a heating system.

Two design aspects dominate collector performance. The first is how efficiently the absorber converts incoming solar radiation into heat while minimizing reflection. The second is how well the collector reduces heat losses by conduction, convection, and radiation to the surrounding environment. Different collector types use glazing, insulation, selective coatings, and sometimes vacuum to address these losses and to reach different operating temperatures.

Main Types Of Solar Thermal Collectors

Several collector designs exist, each suitable for a certain temperature range and application. For beginners it is most useful to understand how they differ in construction, complexity, and typical output temperature.

Flat-Plate Collectors

Flat-plate collectors are the most common type for domestic hot water and low temperature space heating. They consist of a dark, usually selectively coated metal absorber plate, bonded to pipes that carry a liquid. This assembly is placed inside an insulated box with a transparent cover, typically glass, on top.

Sunlight passes through the glass and hits the absorber plate. The plate heats up and transfers heat to the liquid inside the pipes. The glass and insulation reduce heat loss to the environment and create a greenhouse effect inside the box. Flat-plate collectors work best at low to moderate temperature differences between the collector and the ambient air. They are robust, relatively simple to manufacture, and can operate in a wide range of climates, although in very cold regions antifreeze fluids and special designs are used.

Evacuated Tube Collectors

Evacuated tube collectors are designed for higher operating temperatures and better performance in cold climates. Each collector is made up of multiple glass tubes. Inside each tube is an absorber surface, often with a selective coating, and the space between the absorber and the outer glass tube is evacuated, meaning that most air has been removed. The vacuum around the absorber significantly reduces heat loss from convection and conduction.

There are two common internal designs. In one, a U-shaped pipe carrying the heat transfer fluid runs inside each tube. In another, a sealed heat pipe inside each tube carries heat up to a header manifold, where it is transferred to the system fluid. Because of the vacuum, evacuated tube collectors maintain higher efficiencies at greater temperature differences than flat plates. This makes them suitable for applications that need hotter water or for regions with very cold air temperatures.

Unglazed Collectors

Unglazed collectors are simple systems without a glass cover. They often consist of dark plastic or rubber mats or panels through which water flows. Because they lack glazing and heavy insulation, they are inexpensive and have low thermal losses at very small temperature differences, but they also lose heat quickly when the water becomes much warmer than the surrounding air.

These collectors are most commonly used for low temperature applications such as heating swimming pools, where the required water temperature is only slightly above ambient. Their simplicity allows them to be widely installed in suitable climates, but they are not appropriate where high temperatures or freeze protection are needed.

Concentrating Collectors

Concentrating collectors use mirrors or lenses to focus sunlight onto a smaller receiver area, raising the temperature of the working fluid to much higher levels. The basic examples are parabolic troughs, which focus sunlight along a line, and dish systems, which focus onto a point. There are also linear Fresnel reflectors that use long flat or slightly curved mirrors.

Because concentration only works effectively with direct beam sunlight, these collectors are typically deployed in regions with high levels of clear sky radiation. They often operate at temperatures suitable for industrial heat and, in other contexts, for driving thermal power cycles. Their design is more complex, usually involving tracking systems that follow the sun throughout the day to keep the focus on the receiver.

Collector Performance And Temperature Levels

Collectors are often grouped by the temperature range in which they operate effectively. Low temperature collectors such as unglazed pool heaters supply temperatures close to ambient, often up to about 30 or 40 °C. Medium temperature collectors such as typical flat-plate systems provide water or air in the range of about 40 to 80 °C. High temperature collectors such as evacuated tubes and concentrating systems can reach 80 °C and far above, depending on design.

The useful heat output of a collector depends on three main factors. These are the incident solar irradiance on the collector surface, the effective area that intercepts solar energy, and the collector efficiency at the given operating temperature and ambient conditions. A simple way to express this is

$$
Q_{\text{useful}} = \eta \, A \, G
$$

where $Q_{\text{useful}}$ is the useful thermal power output, $\eta$ is the collector efficiency, $A$ is the aperture or absorber area, and $G$ is the solar irradiance on the collector plane.

Because efficiency drops as collectors run hotter, system designers try to match the collector type and size to the required temperature and heat load. Overheating, where the system produces more heat than is used or removed, must be prevented with proper controls and safety features.

Solar Thermal For Domestic Hot Water

One of the most widespread applications of solar thermal collectors is domestic hot water. In this application collectors are installed on roofs or nearby structures and connected to a storage tank. During sunny periods, a pump or natural circulation moves water or a heat transfer fluid through the collectors to absorb heat, which then is stored in the tank.

There are two main hydraulic arrangements. In direct or open-loop systems, potable water flows directly through the collector. This is simple and works in warm climates where freezing is not a concern. In indirect or closed-loop systems, an antifreeze fluid circulates through the collectors and a heat exchanger transfers heat to the potable water in the tank. These systems are preferred in climates with freezing temperatures.

Solar water heating systems are usually designed to cover a significant fraction of annual hot water demand rather than all of it. A backup heater, such as a gas boiler or electric element, complements the solar system on cloudy days or during periods of high demand. Proper system design balances collector area, storage volume, and backup capacity to provide reliability without excessive cost or overheating risk.

Solar Thermal For Space Heating

Solar thermal collectors can also support or provide space heating. Common approaches include using collectors to charge a hot water tank that feeds radiators or underfloor heating loops, or using air collectors that directly warm ventilation air entering a building.

For water-based systems, low temperature distribution systems such as underfloor heating are especially suitable, because they operate effectively with moderate water temperatures, often below 40 or 45 °C. This matches well with flat-plate or evacuated tube collectors. In cold climates, large seasonal variations in solar input and heating demand make fully solar-driven space heating difficult, so solar is typically used for partial coverage. Large storage volumes and careful integration with backup heating are needed to handle daily and weekly fluctuations.

Air-based solar collectors warm air by passing it over an absorber plate. They can be mounted on building facades or roofs. The heated air can either be supplied directly to rooms or routed through ventilation systems. These systems avoid issues such as freezing and fluid leaks, but air carries less heat per unit volume than water, so larger flow rates or collector surfaces are needed.

Industrial And Commercial Process Heat

Many industrial and commercial processes require heat at temperatures from slightly above ambient to several hundred degrees Celsius. Solar thermal collectors can supply part of this heat demand, reducing the need for fossil fuels. The suitability depends on the temperature level required by the process and the available collector technology.

Flat-plate and evacuated tube collectors are typically used for low and medium temperature industrial processes such as washing, cleaning, drying, and preheating of boiler feedwater. Concentrating collectors are applied when higher temperatures are needed, for example in food processing, textiles, or chemical industries where steam or hot fluids at higher pressures are required.

Integrating solar heat into an existing process requires careful consideration of when the process runs, what temperature profiles are needed, and how variable solar input can be handled. Often solar preheats a fluid that is then brought to final process temperature by a conventional boiler. This reduces fuel consumption while maintaining reliability.

Solar Thermal In District Heating Systems

In some regions, large solar thermal fields are connected to district heating networks that supply hot water or steam to multiple buildings. These systems use arrays of collectors, often flat-plate or large-scale evacuated tube installations, feeding heat into a central network.

To balance time mismatches between solar supply and heating demand, large hot water storage tanks or pit storages are used. Seasonal storage systems can accumulate heat over the summer for use in colder months. The scale of these systems makes them significant contributors to urban or community heating needs, particularly where land is available and solar resources are adequate.

Designing solar district heating involves coordinating collector field layout, storage size, network operation, and backup sources such as biomass or natural gas. Efficient distribution pipelines and controls are necessary to maintain required temperatures to consumers while minimizing losses.

Solar Thermal Cooling And Air Conditioning

Though it may seem counterintuitive, solar heat can drive cooling processes. Solar thermal cooling uses heat from collectors to power thermally driven chillers such as absorption or adsorption chillers. These devices use heat to regenerate a working pair of substances that can absorb and release refrigerant, producing chilled water for air conditioning.

Medium and high temperature collectors, such as advanced flat-plate systems, evacuated tubes, or concentrating collectors, are usually required, because many chillers need input temperatures of around 70 °C or higher for efficient operation. Solar cooling is particularly attractive because cooling demand often peaks during sunny periods when solar collectors also perform well.

However, systems are relatively complex. They involve collectors, storage, chillers, and distribution of chilled water. They must be carefully sized and controlled to remain efficient. In some cases, hybrid systems combine solar cooling with conventional electric chillers or with building energy management strategies to improve overall performance.

Solar Thermal For Swimming Pools

Heating swimming pools is a natural match for low temperature solar thermal collectors. The required water temperature is relatively low, often only a few degrees above ambient air temperature. Unglazed plastic collectors are most commonly used for this purpose. They are typically installed on roofs or nearby surfaces and connect directly to the pool circulation system.

Water is diverted from the pool, passed through the collectors where it absorbs heat, and returned to the pool. Controls ensure that water only flows through the collectors when the collector temperature is higher than the pool water, to avoid cooling the pool during unfavorable conditions. Because unglazed systems are inexpensive and simple, they can provide large energy savings compared to fossil fuel pool heaters in suitable climates.

Heat Transfer Fluids And Storage

Solar thermal systems rely on appropriate heat transfer fluids and storage arrangements. Water is the most common fluid but is vulnerable to freezing and scaling. In climates with freezing conditions, mixtures of water and antifreeze, often glycol, are circulated through the collector loop. In indirect systems a heat exchanger separates this loop from potable water.

Storage is crucial for matching the timing of solar heat production and energy use. The simplest storage is a well insulated hot water tank. In solar water heating systems the tank acts as a buffer, allowing heat collected during the day to be used in the evening or early morning. Larger systems may use stratified tanks where hotter water remains at the top and colder water at the bottom, improving efficiency.

In more advanced or large-scale applications, underground pit storages, borehole thermal energy storage, or phase change materials can store heat over longer periods or at more stable temperatures. The choice of storage technology and size strongly influences system performance and the fraction of demand that can be met by solar.

System Integration, Controls, And Safety

Solar thermal collectors and storage must operate as part of a complete system. Pumps, valves, sensors, and controllers work together to optimize performance. A common control logic is differential temperature control. In this arrangement, a controller measures temperatures at the collector outlet and in the storage tank. When the collector temperature exceeds the tank temperature by a set difference, the pump turns on to transfer heat. When the difference falls below a lower threshold, the pump stops to prevent heat loss from the tank back to the collector.

Safety components are essential. Overheating can occur when collectors receive strong sunlight but there is little or no demand and storage is full. Pressure relief valves, expansion vessels, and stagnation management strategies protect the system. In pressurized systems, fluid expansion as it heats can raise pressure. Expansion vessels absorb this extra volume to prevent damage.

Proper installation, insulation of pipes, and regular maintenance maintain efficiency and avoid issues such as leaks, corrosion, air in the system, and degradation of antifreeze properties. System design should always include measures that protect both equipment and users while ensuring reliable heat delivery.

Environmental And Practical Considerations

Solar thermal collectors generally have low operating emissions, since they use sunlight as their energy source. Manufacturing collectors and associated equipment does involve materials and energy, but the thermal output over the lifetime of a system often offsets these initial impacts. Proper design can extend collector lifetimes, often beyond 20 years for many systems.

Practical aspects influence adoption. Roof or ground area must be available and oriented so that collectors receive adequate sunlight with minimal shading. Aesthetic integration into buildings can be a consideration, especially in urban or historic areas. In some cases, façade mounted collectors or architectural integration can address these concerns.

Finally, local climate, energy prices, and support policies affect the economic attractiveness of solar thermal applications. Areas with high solar resources and significant hot water or heat demand, such as hotels, hospitals, and industrial facilities, can particularly benefit. At the household level, solar water heaters can reduce energy bills and reliance on fossil fuels, contributing to more sustainable energy use.