Solar thermal energy (STE) is a type of energy and a method for capturing sunlight to create heat. This heat is used in industries, homes, and businesses. The United States Energy Information Administration divides solar thermal collectors into three groups: low-, medium-, and high-temperature collectors. Low-temperature collectors usually do not have a glass cover and are used to warm swimming pools or air for buildings. Medium-temperature collectors are often flat plates and are used to heat water or air for homes and businesses.

High-temperature collectors use mirrors or lenses to focus sunlight. These collectors are used in industries to provide heat up to 300 °C (600 °F) or 20 bar (300 psi) pressure. They are also used to generate electricity. Two types of high-temperature systems are Concentrated Solar Thermal (CST), which provides heat for industries, and concentrated solar power (CSP), which uses heat to create electricity. CST and CSP are used for different purposes and cannot replace each other.

Unlike photovoltaic cells, which turn sunlight directly into electricity, solar thermal systems use sunlight to produce heat. These systems use mirrors or lenses to focus sunlight onto a receiver, which heats water. The hot water can then be used in homes. A benefit of solar thermal systems is that the heated water can be stored until it is needed, so no extra storage system is required. Solar thermal power can also be used to create electricity by turning heated water into steam, which drives a turbine connected to a generator. However, this method of producing electricity is more expensive than photovoltaic power plants, so very few are used today.

History

Augustin Mouchot showed a device that collects sunlight and a machine that makes ice cream at the 1878 Universal Exhibition in Paris. Frank Shuman built the first solar thermal system in the Sahara around 1910. He used sunlight to create steam, which powered a steam engine. Later, liquid fuel engines became more popular because they were easier to use. This caused the Sahara project to be stopped. It was later looked at again after many years. As of 2023, the largest thermal solar power plant in the world is located in the United Arab Emirates.

Low-temperature heating and cooling

Systems that use low-temperature solar thermal energy include ways to collect heat, store it for short or long periods, and move it to a building or a heating network. Sometimes one part of the system can do more than one job, like certain solar collectors that also store heat. Some systems work without extra energy, while others need outside power to operate.

Heating is the most common use of solar energy, but cooling can also be done using special machines called absorption or adsorption chillers. These machines use heat from the sun to make cold air for buildings or groups of buildings. In 1878, Auguste Mouchout created a system that used solar energy to make ice with a steam engine and a refrigeration device.

In the United States, heating, ventilation, and air conditioning (HVAC) systems use more than 25% of the energy in commercial buildings and nearly half of the energy in homes. Solar heating, cooling, and ventilation systems can help reduce this energy use. The most common solar heating system for buildings is called a transpired solar air collector, which connects to a building’s HVAC system. As of 2015, over 500,000 square meters of these panels were in use in North America.

In Europe, about 125 large solar-thermal heating plants were built between the mid-1990s and now. Each has more than 500 square meters of solar collectors. The largest plants are about 10,000 square meters, with the ability to produce 7 megawatts of heat and costs around 4 cents per kilowatt-hour without help from the government. Forty of these plants can produce at least 1 megawatt of heat. A program called Solar District Heating (SDH) includes 14 European countries and the European Commission. It works to improve technology and market growth and holds yearly meetings.

Glazed solar collectors are mainly used for heating buildings. They move air from inside a building through a solar panel where the air is warmed and then sent back inside. These systems need at least two openings in the building and only work when the air in the solar panel is warmer than the air inside the building. Most glazed collectors are used in homes.

Unglazed solar collectors are used to warm the air that enters buildings through ventilation systems, especially in places with high air flow needs like schools, factories, and offices. These collectors are often painted metal panels attached to building walls. They heat air by absorbing sunlight on their surface and then transfer that heat to the air inside the building. The heated air is pulled into the building’s ventilation system through small holes in the wall.

A Trombe wall is a passive solar heating and ventilation system. It has a channel of air between a window and a wall that stores heat from the sun. During the day, sunlight warms the wall, which then heats the air in the channel. This causes air to move through vents at the top and bottom of the wall. At night, the wall releases the stored heat into the building.

Solar roof ponds were created in the 1960s by Harold Hay. A basic system includes a water container on a roof with a cover that can be opened or closed. During the day, the cover is removed to let sunlight heat the water, which stores heat for use at night. When cooling is needed, the cover is closed during the day to collect heat from inside the building and opened at night to release the heat to the cooler air outside. A building called the Skytherm House in California uses a prototype of this system.

In the United States and Canada, solar air heat collectors are more commonly used for heating than solar liquid collectors because most buildings already have ventilation systems. The two main types of solar air panels are glazed and unglazed.

In 2007, the United States produced 21,000,000 square feet of solar thermal collectors, with 16,000,000 square feet being low-temperature collectors. These are often used to heat swimming pools but can also be used for building heating. Collectors can use air or water to move heat to where it is needed. Solar energy can also be used to heat water for daily use, like the hot water from taps. Solar water heating systems can provide about half of a home’s hot water needs each year, depending on the size of the home and where it is located. This can help save money on energy bills.

Heat storage for space heating

Seasonal thermal energy storage (STES) is a collection of advanced technologies that can store heat for months. This allows solar heat collected in the summer to be used for heating throughout the year. STES technology has been developed mainly in Denmark, Germany, and Canada. It is used in individual buildings and district heating systems. For example, Drake Landing Solar Community in Alberta, Canada, has a small district heating system. In 2012, this community reached a world record by meeting 97% of its heating needs using solar energy. STES uses different materials to store heat, such as deep aquifers, natural rock around small holes with heat exchangers, large shallow pits filled with gravel and insulated, and large insulated tanks buried underground.

Centralized district heating systems can operate continuously using concentrated solar thermal (CST) storage plants.

Interseasonal storage allows solar heat or heat from other sources to be stored between opposite seasons. This is done in aquifers, underground rock layers, specially built pits, and large insulated tanks covered with earth.

Short-term storage uses materials like stone, concrete, and water to store solar energy during the day and release it during cooler times. The amount and placement of these materials depend on factors such as climate, sunlight exposure, and shading. When used properly, these materials can help maintain comfortable temperatures and reduce energy use.

By 2011, about 750 cooling systems powered by solar heat pumps existed worldwide. These systems grew by 40% to 70% each year for seven years. However, this is a small market because it is expensive to operate, and the number of hours cooling is needed varies by region. For example, cooling is needed about 1,000 hours a year in the Mediterranean, 2,500 hours in Southeast Asia, and only 50 to 200 hours in Central Europe. Between 2007 and 2011, the cost to build these systems dropped by about half. The International Energy Agency (IEA) Solar Heating and Cooling program (IEA-SHC) continues to work on improving these technologies.

A solar chimney is a passive solar ventilation system that uses a hollow structure connecting a building’s interior and exterior. When the chimney warms, the air inside rises, creating airflow that pulls cooler air into the building. These systems have been used since Roman times and are still common in the Middle East.

Solar process heating systems are designed to provide large amounts of hot water or heating for nonresidential buildings.

Evaporation ponds are shallow pools that use the sun to remove water through evaporation, leaving behind dissolved solids. This method has been used for thousands of years to extract salt from seawater. Modern uses include concentrating brine for mining and removing solids from waste. Evaporation ponds are one of the largest commercial uses of solar energy today.

Unglazed transpired collectors are walls with holes that face the sun and are used to preheat air for buildings. These collectors can be mounted on roofs and used year-round. They can raise incoming air temperatures up to 22°C (72°F) and produce air temperatures between 45–60°C (110–140°F). These systems pay for themselves quickly, often in 3 to 12 years, making them more cost-effective than glazed systems. By 2015, over 4,000 of these systems had been installed globally, covering 500,000 square meters. Examples include a 860-meter (9,300-foot) collector in Costa Rica used for drying coffee and a 1,300-meter (14,000-foot) collector in Coimbatore, India, used for drying marigolds.

A food processing facility in Modesto, California, uses parabolic troughs to produce steam for its manufacturing processes. The 5,000 square meter collector area is expected to generate 15 terajoules of energy each year.

Medium-temperature collectors

These collectors can help provide about 50% or more of the hot water needed for homes and businesses in the United States. In the United States, a typical system costs $4000–$6000 at retail ($1400 to $2200 for materials at wholesale). About 30% of the system qualifies for a federal tax credit, and some states offer additional credits. Installing a simple open loop system in southern areas takes 3–5 hours, while installation in northern areas takes 4–6 hours. Northern systems require more collectors and more complex plumbing to prevent freezing. With these incentives, a typical household can save money in 4 to 9 years, depending on the state. Similar support exists in parts of Europe. A team of one solar plumber and two helpers with basic training can install a system in one day. Thermosiphon systems have very low maintenance costs, but costs increase if antifreeze or electricity is used for circulation. In the United States, these systems reduce a household's monthly operating costs by $6 per person. Solar water heating can reduce CO₂ emissions by 1 ton per year for a family of four if it replaces natural gas, or 3 tons per year if it replaces electricity. Medium-temperature systems can use several designs, including pressurized glycol, drain back, batch systems, and newer low-pressure freeze-tolerant systems with polymer pipes and photovoltaic pumping. Standards for medium-temperature collectors are being updated to include new designs and operations. One innovation is "permanently wetted collector" systems, which reduce or eliminate high-temperature stress that can damage collectors.

Solar thermal energy can be used to dry wood for construction and wood fuels like wood chips. It is also used to dry food products such as fruits, grains, and fish. Solar drying is environmentally friendly and cost-effective, improving product quality. Lower production costs allow products to be sold at lower prices, benefiting buyers and sellers. Technologies for solar drying include low-cost air collectors based on black fabric. Solar thermal energy helps dry wood chips and other biomass by increasing temperature and allowing air to remove moisture.

Solar cookers use sunlight to cook, dry, and pasteurize food. They reduce fuel costs, lower the need for firewood, and improve air quality by reducing smoke. The simplest solar cooker is the box cooker, first created by Horace de Saussure in 1767. A box cooker has an insulated container with a transparent lid. These cookers work well even on partly cloudy days and can reach temperatures of 50–100 °C (100–200 °F).

Concentrating solar cookers use reflectors to focus sunlight onto a cooking container. Common reflector shapes include flat plates, discs, and parabolic troughs. These designs cook faster and at higher temperatures (up to 350 °C; 660 °F) but need direct sunlight to work.

The Solar Kitchen in Auroville, India, uses a unique concentrating technology called the solar bowl. Unlike traditional systems, the solar bowl uses a fixed spherical reflector with a receiver that tracks sunlight as it moves. This system reaches temperatures of 150 °C (300 °F) to produce steam for cooking 2,000 meals daily.

Many solar kitchens in India use another technology called the Scheffler reflector, developed by Wolfgang Scheffler in 1986. A Scheffler reflector is a curved dish that follows the sun’s path using single-axis tracking. Its flexible surface adjusts to seasonal sunlight changes. These reflectors have a fixed focal point, making cooking easier, and can reach temperatures of 450–650 °C (850–1200 °F). The largest Scheffler system, built in 1999 by Brahma Kumaris in Rajasthan, India, can cook up to 35,000 meals daily. By 2008, over 2,000 Scheffler cookers had been built globally.

Solar stills can produce drinking water in areas without clean water. Solar distillation heats water in the still, causing it to evaporate and condense on the glass cover.

Dr. Lin Zhao of MIT published a study in the journal Joule describing a solar autoclave that sterilizes surgical tools without electricity. A prototype using low-cost aerogel was tested successfully at a hospital in Mumbai with help from IIT Bombay.

High-temperature collectors

Flat-plate collectors are often used for space heating when temperatures below about 95 °C (200 °F) are enough. These collectors lose a lot of heat through their glass covering, so they cannot reach temperatures much higher than 200 °C (400 °F), even if the heat transfer fluid is not moving. These temperatures are too low to make electricity efficiently.

The efficiency of heat engines improves as the temperature of the heat source increases. To achieve higher temperatures in solar thermal plants, mirrors or lenses are used to focus sunlight, a method called Concentrated Solar Power (CSP). This increases efficiency, reduces the size of the plant’s collectors, and lowers land use and costs.

At temperatures up to 600 °C (1100 °F), steam turbines are commonly used and can reach an efficiency of about 41%. Above 600 °C (1100 °F), gas turbines may be more efficient. Very high temperatures, such as 700 °C (1300 °F) to 800 °C (1500 °F), can be achieved using liquid fluoride salts and multi-stage turbines, which may reach 50% efficiency. Higher temperatures allow plants to use dry heat exchangers, reducing water use, and improve heat storage efficiency by storing more energy per unit of fluid.

Commercial CSP plants were first developed in the 1980s. The largest solar thermal plants today are the 370 MW Ivanpah Solar Power Facility (built in 2014) and the 354 MW SEGS CSP installation, both in California’s Mojave Desert.

A major advantage of CSP is its ability to store thermal energy, allowing electricity to be produced for up to 24 hours. Since electricity demand is highest in the late afternoon, many CSP plants use 3 to 5 hours of thermal storage. Storing heat is cheaper and more efficient than storing electricity, enabling CSP plants to generate power day and night. If solar radiation is predictable, CSP plants become reliable. Reliability can improve further with backup systems that use existing CSP equipment, reducing costs.

CSP’s main challenges include cost, land use, and the need for high-voltage transmission lines. Although solar power uses less land than fossil fuels for the same energy output, large areas are still needed to collect enough sunlight. Using simple designs helps reduce costs.

Compared to fossil fuels, utility-scale solar power uses significantly less land. For example, the federal government has allocated nearly 2,000 times more land for oil and gas leases than for solar projects. In 2010, the Bureau of Land Management approved nine large-scale solar projects covering 40,000 acres, while approving over 5,200 oil and gas lease applications covering 3.2 million acres.

During the day, the sun’s position changes. For systems that do not require high temperatures, tracking the sun’s movement can be avoided using nonimaging optics. For systems that focus sunlight intensely, tracking systems are needed to keep the light focused on the receiver. Tracking systems increase cost and complexity, so different designs are used based on how they concentrate light and track the sun.

Parabolic trough power plants use curved, mirrored troughs to reflect sunlight onto a glass tube containing a fluid (receiver). The trough is curved in one direction and straight in the other. To follow the sun’s daily movement, the trough tilts east to west. Seasonal changes in sunlight angle do not require mirror adjustments because the light is still focused on the receiver. The receiver may be inside a glass vacuum chamber to reduce heat loss.

The hot fluid inside the receiver is transported to a heat engine, where about one-third of the heat is converted to electricity. Full-scale parabolic trough systems use many troughs arranged in parallel over large areas. Since 1985, the SEGS system in California has operated using this technology. Other CSP designs lack the same level of proven experience, making parabolic troughs the most tested CSP technology.

The SEGS system includes nine plants with a total capacity of 354 MW and has been the world’s largest solar power plant for many years. A newer plant, Nevada Solar One, has a capacity of 64 MW. Spain’s Andasol solar power stations (150 MW total) use heat storage, allowing them to operate during the day and some nighttime hours. This storage system helps Andasol produce more energy than Nevada Solar One despite having a smaller peak capacity. The 280 MW Solana Generating Station in Arizona, which came online in 2013, has 6 hours of power storage. Other plants, like Hassi R'Mel in Algeria and Martin Next Generation Solar Energy Center, use parabolic troughs in combination with natural gas.

Parabolic trough systems are often enclosed in greenhouse-like glasshouses to protect the equipment from weather conditions that might reduce reliability or efficiency. Lightweight, curved mirrors are suspended inside the structure, and a single-axis tracking system adjusts their position to follow the sun’s movement.

Heat collection and exchange

Heat in a solar thermal system follows five basic principles: heat gain, heat transfer, heat storage, heat transport, and heat insulation. Heat is a measure of how much thermal energy an object has, based on its temperature, mass, and specific heat. Solar thermal power plants use heat exchangers that work well under constant conditions to move heat. Copper is important in these systems because it conducts heat quickly, resists damage from air and water, can be sealed and joined easily, and is strong. Copper is used in parts like receivers and pipes in solar thermal water systems.

Heat gain is the heat collected from the sun by the system. Solar thermal heat is captured using the greenhouse effect, which allows short wave sunlight to pass through a reflective surface while reflecting longer wave heat radiation. When sunlight hits an absorber plate, it creates heat and infrared radiation, which is trapped inside the collector. Fluid, such as water, in the absorber tubes collects this heat and moves it to a heat storage vault.

Heat moves through conduction or convection. When water is heated, energy is passed to other water molecules through conduction. These molecules spread their heat energy and take up more space than colder molecules. As hot water rises and cold water sinks, convection helps move heat. Heat from the collector’s absorber plates is transferred to the fluid through conduction. The fluid is then moved through pipes to the heat storage vault, where convection spreads the heat further.

Heat storage allows solar thermal plants to generate electricity even when there is no sunlight. During sunny hours, heat is stored in an insulated reservoir. When sunlight is unavailable, heat is removed from the reservoir to create energy. How quickly heat moves depends on the material’s ability to conduct or convect heat and the temperature difference between objects. Objects with larger temperature differences transfer heat faster.

Heat transport is the process of moving heat from a solar collector to a storage vault. Heat insulation is important in both the pipes used for transport and the storage vault. It prevents heat loss, which reduces energy waste and keeps the system efficient.

Heat storage for electric base loads

Heat storage helps solar thermal plants make electricity at night and on cloudy days. This allows solar power to supply electricity continuously and during times of high demand, which could replace power from coal and natural gas plants. Using heat storage also makes generators work more efficiently, which lowers costs. Even short-term storage can help balance the sudden changes in electricity needs at sunset when a grid has a lot of solar power.

During the day, heat is stored in an insulated container using a special material. This material can be pressurized steam, concrete, phase change materials, or molten salts like calcium, sodium, and potassium nitrate. At night, the stored heat is used to make electricity.

The PS10 solar power tower stores heat in tanks as pressurized steam at 50 bar (700 psi) and 285°C (545°F). When pressure is reduced, the steam condenses and turns back into steam. The storage lasts for one hour. Longer storage is possible, but it has not been tested in a working power plant.

Molten salt is used in solar power towers because it is liquid at normal air pressure, stores heat at a low cost, works well with steam turbines, and is not flammable or toxic. It is also used in chemical and metal industries to move heat.

The first commercial molten salt mix was 60% sodium nitrate and 40% potassium nitrate. This salt melts at 220°C (430°F) and stays liquid at 290°C (550°F) in an insulated tank. Adding calcium nitrate lowers the melting point to 131°C (268°F), allowing more energy to be used before freezing. Some calcium nitrate grades can stay stable above 500°C (1000°F).

This solar system can make electricity on cloudy days or at night using stored heat in hot salt tanks. The tanks are insulated and can keep heat for up to a week. Tanks that power a 100-megawatt turbine for four hours would be about 9 meters (30 feet) tall and 24 meters (80 feet) wide.

The Andasol power plant in Spain was the first commercial solar thermal plant to use molten salt for heat storage and nighttime electricity. It started operating in March 2009. In July 2011, Torresol's 19.9 MW solar plant in Spain became the first to produce electricity nonstop for 24 hours using molten salt storage.

In January 2019, the Shouhang Energy Saving Dunhuang 100MW molten salt tower project in China started operating. It includes an 11-hour heat storage system and can produce electricity for 24 hours straight.

Phase Change Materials (PCMs) are another option for storing energy. Like molten salt, PCMs use similar heat transfer systems but may store energy more efficiently. PCMs can be organic or inorganic. Organic PCMs are not corrosive, have stable chemical and thermal properties, and do not cool too quickly. However, they have lower energy storage capacity and are flammable. Inorganic PCMs store more energy but may cool too quickly, cause corrosion, or break down over time. Because inorganic PCMs store more energy, hydrate salts are a good choice for solar energy storage.

Use of water

Solar thermal power plants that need water for cooling or condensation may face challenges when built in desert areas with limited water supplies but high solar energy potential. For example, Solar Millennium, a German company, planned to build a plant in the Amargosa Valley of Nevada. This project would use 20% of the water available in the area. Similar projects in California's Mojave Desert may also struggle due to difficulty in securing proper water rights. California law currently prevents the use of drinking water for cooling purposes.

Other designs use less water. The Ivanpah Solar Power Facility in southeastern California reduces water use by using air-cooling to change steam back into water. This method uses 90% less water than traditional wet-cooling, though it slightly lowers efficiency. The water is then reused in a closed system, which is better for the environment.

Electrical conversion efficiency

Among these technologies, the solar dish/Stirling engine has the highest energy efficiency. A single solar dish-Stirling engine at Sandia National Laboratories' National Solar Thermal Test Facility (NSTTF) can produce up to 25 kilowatts of electricity, with an efficiency of 31.25%.

Solar parabolic trough plants typically operate at about 20% efficiency. Fresnel reflectors have slightly lower efficiency, but they can be packed more densely.

Overall efficiency is calculated by comparing the total electricity generated to the amount of sunlight that hits the entire area of the solar power plant. For example, the 500-megawatt (MW) SCE/SES plant uses about 2.75% of the sunlight that falls on its 4,500-acre (18.2 km²) area. The 50 MW AndaSol Power Plant in Spain, which covers 1.95 km² (3⁄4 square mile), has an overall efficiency of 2.6%.

Efficiency does not directly affect cost. Total cost includes expenses for building and maintaining the system.