Table of Contents
Introduction
Solar energy reaches Earth in the form of sunlight and can be used in two main technical ways. One way turns sunlight directly into electricity, the other captures the sun’s heat. These two families of technologies are called solar photovoltaic and solar thermal. Both use the same solar resource, but they work differently, serve different types of energy needs, and fit into projects in distinct ways.
Basic Idea Of Solar Photovoltaic
Solar photovoltaic, often shortened to PV, uses special materials that create an electric current when light falls on them. In PV systems, sunlight is converted directly into electrical energy in the form of direct current. This electricity can then be used immediately, stored in batteries, or converted to alternating current to power homes, businesses, or feed into the grid.
PV does not rely on the temperature of the air or the physical heating of a fluid. What matters most is the amount of sunlight that reaches the modules, especially in the visible and near infrared parts of the spectrum. PV modules produce electricity as long as there is light, even on cold or cloudy days, although output is lower when light is weaker.
Basic Idea Of Solar Thermal
Solar thermal technologies use sunlight to heat a working medium such as water, air, or special oils. The core product is useful heat, not electricity, although some solar thermal power plants do produce electricity by driving a turbine with heated fluid.
In simple solar thermal systems, such as domestic solar water heaters, sunlight warms water for showers or space heating. In more complex systems like solar process heat for industry or concentrated solar power plants, mirrors or lenses focus sunlight to reach much higher temperatures. The heated medium can then be used directly for heating or to generate steam that drives a turbine to produce electricity.
Direct Electricity Versus Heat
The most fundamental difference between solar PV and solar thermal is the kind of energy output they provide. PV systems deliver electricity directly, which is a highly flexible energy form that can power lights, appliances, motors, electronics, and can be easily transported over long distances through power lines.
Solar thermal systems deliver heat. This can be low temperature heat for hot water or space heating, medium temperature heat for industrial processes, or high temperature heat for power generation. Heat is often best used close to where it is produced, because moving large amounts of hot fluid over long distances is usually less efficient and more costly than transmitting electricity.
This difference in energy form strongly shapes where each technology is most useful. When the main need is electrical power, PV is generally the first choice. When the main need is hot water, space heating, or industrial heat, solar thermal can be more suitable and efficient.
System Complexity And Components
PV systems are mechanically simple. The key components are PV modules, mounting structures, cabling, and power electronics such as inverters and sometimes batteries. PV has no moving parts in the modules themselves and does not need pumps or complex piping. This relative simplicity makes PV attractive for many building rooftops and small-scale applications.
Solar thermal systems usually involve more mechanical components. Even basic solar water heaters commonly have collectors, insulated piping, pumps or thermosiphon arrangements, storage tanks, control valves, and insulation. Larger or higher temperature systems require more complex designs, including tracking mirrors, heat transfer fluids, heat exchangers, and thermal storage units. The added complexity can increase installation and maintenance needs but can also enable more control over temperature levels and storage.
Efficiency And Performance Considerations
There are different kinds of efficiency to consider. For PV, the key figure is the fraction of incoming solar energy that a module converts to electrical energy. Commercial PV modules often have conversion efficiencies in the range of roughly 15 to 25 percent. This means that only a part of the sunlight is turned into electricity, while the rest mainly becomes heat.
For solar thermal collectors, especially for low temperature uses, the situation is different. Collectors can capture a much larger fraction of the incoming solar energy as useful heat, often over 50 percent under suitable conditions. Because creating low temperature heat is less demanding than producing electricity, solar thermal can appear more efficient when the need is simply hot water or space heating.
However, the usefulness of the output matters just as much as the numerical efficiency. Even if PV has a lower percentage efficiency, its electrical output can be more valuable for certain applications because it can run a wide range of devices and can be fed into the grid.
A key practical rule is: use solar thermal when you mainly need heat, and use solar PV when you mainly need electricity.
Storage Options And Flexibility
Both technologies must cope with the variability of sunlight. The sun does not shine at night and is weaker on cloudy days. How storage works is an important difference between solar thermal and PV.
Solar thermal systems naturally connect to thermal storage. Hot water tanks or insulated thermal reservoirs can store solar heat for several hours or sometimes days at relatively low cost. For heating-focused applications, this can smooth out day and night differences.
PV electricity is usually stored in electrochemical batteries if storage is required onsite. Batteries are often more expensive per unit of stored energy than simple hot water tanks, but they offer high flexibility, fast response, and can supply electricity on demand. In many grid-connected systems, the grid itself acts as a kind of indirect storage, because surplus PV electricity can be fed in and drawn back at other times under suitable policies and tariffs.
Temperature Levels And Applications
Solar thermal and solar PV differ in the temperature ranges that they deal with. PV modules operate within a certain temperature range for good performance, but they do not intentionally deliver heat. In fact, higher module temperatures usually reduce PV electrical output.
Solar thermal collectors are designed to capture and use rising temperatures. Flat plate or evacuated tube collectors are suitable for low to medium temperatures, such as domestic hot water or space heating. Concentrating solar systems that use mirrors or lenses can reach higher temperatures for industrial heat or power generation.
Because of this, solar thermal is especially strong where there is a clear need for hot water or process heat, such as in households, hotels, hospitals, and certain industries. In contrast, PV is especially strong where there is a need for general electrical power for devices and equipment.
Cost And Market Trends
Costs have evolved differently for solar PV and solar thermal. The price of PV modules has fallen significantly over the last decades due to large-scale manufacturing, technology improvements, and global deployment. This has driven the rapid expansion of PV systems of all sizes.
Solar thermal technologies, while mature for basic applications like domestic water heating, have not seen the same level of global cost reduction and standardization as PV in many regions. In some markets, cheap PV electricity combined with electric water heaters or heat pumps has started to compete strongly with traditional solar thermal water heaters.
However, in locations with high fuel prices and strong demand for hot water, or where policies support solar heat, solar thermal can remain very cost effective. For higher temperature industrial uses, solar thermal can also be attractive, especially when compared to burning fossil fuels for process heat.
Space Requirements And Integration In Buildings
On a rooftop or a piece of land, PV and solar thermal use space differently. For the same roof area, well-designed solar thermal systems can deliver more usable heat energy for water heating than PV can deliver electricity that is then converted to heat. This makes solar thermal space efficient for direct heating uses.
For general electricity needs, PV has the advantage of being modular and easy to integrate. PV modules can be mounted on roofs, façades, parking structures, or integrated into building elements. In many modern projects, designers decide whether to prioritize rooftop area for PV, for solar thermal, or for a mix of both, depending on the local climate, energy prices, and the building’s heating and electrical demands.
Environmental And Operational Differences
Both solar PV and solar thermal avoid direct fuel combustion during operation and therefore do not emit greenhouse gases on site during normal use. Their broader environmental impacts relate to manufacturing, installation, maintenance, and end of life, which are treated elsewhere in the course.
Operationally, solar thermal systems need attention to freezing risks, overheating, leaks, and corrosion in piping and tanks. PV systems mainly require electrical checks, cleaning of modules when necessary, and monitoring of power electronics performance.
In cold climates, properly designed solar thermal systems use antifreeze solutions or drain-back designs to prevent damage. PV systems, by contrast, can even benefit from cooler ambient temperatures in terms of electrical efficiency. In very hot climates, PV output can drop due to heat, while solar thermal output can increase but may require careful control to avoid overheating.
Choosing Between Solar Thermal And Solar PV
Selecting between solar thermal and solar photovoltaic is rarely a purely technical question. It depends on the energy services needed, the existing infrastructure, local climate, building design, available space, and economics.
When a project is focused on heating large amounts of water, such as in multi family housing, swimming pools, hotels, or certain industries, solar thermal often provides a direct and efficient solution. When the project aims to reduce electricity bills, power equipment, or contribute to the grid, solar PV is usually preferred.
In many cases, a combination works well. For example, a building might use solar thermal panels for domestic hot water and PV modules for electricity. As PV costs continue to fall and electric heat pump technologies spread, some projects find that using PV electricity to power heat pumps can substitute for traditional solar thermal systems. Other projects, especially with high and constant heat demand, still favor dedicated solar thermal solutions.
The most suitable technology is the one that best matches the main energy demand of the user, taking into account whether that demand is primarily for heat or for electricity.
Summary
Solar photovoltaic and solar thermal both capture energy from the sun but deliver it in different forms and through different technical means. PV produces electricity directly from light, while solar thermal collects heat. They differ in system complexity, efficiency, storage, temperature ranges, costs, and typical uses. Understanding these differences helps match the right solar technology, or combination of technologies, to specific energy needs in homes, businesses, and industry.