Solar PV vs. Solar Thermal — What’s the Difference?
Quick Answer: Solar PV and solar thermal both harness energy from the sun but for different purposes. Photovoltaic (PV) systems convert sunlight directly into electricity, while thermal systems produce thermal energy for residential heating systems such as hot water or space heaters.
The differences also come down to how they capture energy from sunlight. PV systems generate electricity when photovoltaic panels capture solar energy and convert it into DC electricity. Thermal systems capture the sun’s heat through thermal panels that absorb the sun’s thermal energy and transmit it to a heat-transfer fluid.
In this article, you’ll learn:
- The differences between solar photovoltaics and thermal energy systems;
- How a photovoltaic panel converts sunlight into electricity;
- The different types of solar thermal systems, including flat-plate collectors and evacuated-tube collectors;
- Which system is best for your energy needs.
Solar Photovoltaic
Solar photovoltaic (PV) technology is a renewable energy system that converts sunlight into electricity via solar panels. A PV panel contains photovoltaic cells, also called solar cells, which convert light photons (light) into voltage (electricity). This phenomenon is known as the photovoltaic effect.
How Does Solar Photovoltaic Work?
Photovoltaic panels consist of semiconductor materials (usually silicon). When sunlight strikes the surface of a PV panel, the semiconductor absorbs energy from the photons. That reaction releases electrons from their atomic bonds. It creates a flow of electrons, resulting in an electric current.
The generated electric current is in the form of a direct current (DC). An inverter converts the DC power into alternating current (AC) to make this electricity usable for most household appliances and the electrical grid.
Components of Solar Photovoltaic (PV) System
PV systems have various interconnected components that work together to provide electricity to your home. These components include:
- Photovoltaic Panels
- Charge Controller
- Solar Battery Bank
- Inverter
- Utility Meter
- Electric Grid
Off-grid systems only use the first four components, as they do not utilize utility meters or electric grids.
The solar panels are your system’s first (and most important!) component. They interface directly with the sun’s rays, converting the photons into electricity.
An inverter converts direct current (DC) electricity into alternating current (AC) electricity. The inverter is crucial since PV panels produce DC electricity, while most household appliances and electrical systems operate on AC. Common types of inverters include string inverters, microinverters, and hybrid inverters.
The charge controller comes next in a PV system. This device sits between the photovoltaic panels and batteries to regulate the electricity that passes between them. The charge controller prevents overcharging and transmits an electrical current to the battery bank.
A battery bank stores electricity for later use. Also called a solar battery, it is handy for cloudy days or wintertime when your PV array produces less power.
Utility meters are an essential part of any grid-tied system. These devices measure the flow of electricity between the electric grid and your home’s solar system. A utility meter will track the electricity produced and consumed by a home.
The electric grid is the final component of a grid-tied system. The power produced from a residential solar array is sent through a utility meter and out into the electric grid. When a home draws power, it also pulls from the electric grid (unless the system has an energy storage system like a battery).
Net metering programs allow homeowners to receive payment for any excess energy produced during a billing period.
Advantages
- Whole-Home Power — provides electricity to an entire house, including appliances, lights, water heating, HVAC, and more.
- Renewable Energy — PV systems are a renewable energy source, reducing the user’s reliance on fossil fuels and utility companies.
- Lower Electricity Bills — photovoltaics can drastically reduce or eliminate your monthly electricity bill.
Disadvantages
- Upfront Cost — upfront costs for a PV system can be in the tens of thousands. However, this cost is easily recoupable over the system’s lifetime.
- Aesthetics — some home and business owners may find a solar array visually unappealing.
- Space Requirements — whole-home PV systems require around 480 sq ft (45 M2 on average.)
Solar Thermal
Solar thermal panels perform a similar function to PV panels by converting sunlight into usable energy. However, thermal panels differ in that they use a heat-transfer fluid — either water or air — to capture the energy, as opposed to the semiconductors of PV panels.
Thermal systems are an efficient and environmentally friendly method for residential or commercial heating. They reduce the user’s dependency on fossil fuels and lower greenhouse gas emissions.
How Does Solar Thermal Work?
Depending on the intended usage, there are a few different types of thermal systems. In all solar thermal systems, a heat-transfer fluid (water or air) collects energy from the sun. The hot fluid is then used directly in the space for heating, or it can produce steam for mechanical energy.
Most residential systems use flat-plate collectors. The thermal panel consists of a dark, flat surface encased in a thermally-insulated box. The dark colour of the panel allows more energy absorption.
Another common type of thermal system is the evacuated tube collector. This type of panel features a series of glass tubes containing a vacuum, which reduces energy loss.
Either of these panel types can work for hot water heating, space heating, or electricity generation.
For home heating, the heat-transfer fluid can circulate through pipes in a floor through radiant heating. A radiant floor system radiates the heat from the liquid into the room.
Solar thermal systems can also operate on a commercial scale for energy production. The heat-transfer fluid produces steam that, when passed through a turbine, powers a generator that produces electricity.
Components Used in a Solar Thermal System
While individual systems will vary, a few components are common to most thermal systems.
Solar thermal collectors are the “panels” in a thermal system. They are usually installed on a home’s roof and convert the sun’s energy into heat.
The heat transfer fluid flows through a thermal collector and transfers the heat to the rest of the system.
The pump station distributes the heat transfer fluid throughout the system.
A controller monitors and regulates the transfer process. It controls the other system components, ensuring safe and reliable operation.
A hot water tank will likely be integrated into the design if the thermal system is for heating household water. For radiant heating systems, pipes installed in the floor allow the heat transfer fluid to flow throughout the home.
Advantages
- Efficiency — thermal systems are an efficient way to convert sunlight into heat for your home.
- Renewable Energy — a thermal system utilizes renewable energy to reduce environmental impact.
- Reduced Heating Bills —a thermal system may reduce your monthly water, gas, or electricity bill.
Disadvantages
- Limited Use — thermal systems are not practical for whole home electricity generation. A photovoltaic system is more efficient for this purpose.
- Incompatibility — radiant heating systems are part of the construction process, installed before pouring the concrete foundation for a home. It is often impractical to retrofit a home with radiant heating.
- Intermittent Energy Generation — a thermal system will only function while sunlight is available, so your energy production may decrease depending on the weather, season, or time of day.
Concentrated solar thermal research
We are leading the way in concentrated solar thermal research, specialising in high-temperature central receiver systems.
Focusing sunlight to generate heat
Concentrated solar thermal (CST) technology concentrates sunlight onto a target to create very high temperatures. This heat can be used directly in industrial processes or to generate electricity by heating water for steam to turn a turbine.
Harnessing renewable energy to decarbonise Australia’s industry is one of our biggest challenges. As lowering emissions becomes essential for industry and the community, we are looking at new ways of generating thermal energy from sunlight. Our challenge is how to make this solar a reliable, stable part of Australia’s energy future.
Heliostats are sun-tracking mirrors that concentrate sunlight by focusing it onto a target, generating temperatures of hundreds of degrees. In a heliostat field, a central receiver system or ‘power tower’ is used to harness the heat of the sun.
Rather than letting it radiate onto the ground, each heliostat magnifies solar radiation by focussing the reflected energy into a small focal point on the tower. A large number of focused heliostats produces a tremendous amount of heat.
Our response
Creating advanced solar thermal systems
Although many commercial CST power stations are already in operation overseas, research is needed to lower the cost of CST technology. We aim to make electricity from CST competitive with fossil fuel-generated electricity in Australia through the Australian Solar Thermal Research Institute (ASTRI).
Our Energy Centre in Newcastle contains the only high-temperature solar thermal research facility of its type in Australia, home to the largest high-concentration solar array in the Southern Hemisphere.
The site has two facilities: Solar Field 1 and Solar Field 2. Both are operated from an elevated control room housing the centre’s communications and control systems. Each field contains a tower and a heliostat array that tracks the sun throughout the day, concentrating the solar heat to produce temperatures over 1,000 degrees Celsius (ºC).
Our CST demonstration and research facilities have been used to:
- ‘supercharge’ natural gas (SolarGas)
- store energy, so that solar power can be used when it’s cloudy or after dark
- generate electricity from the sun and air in a solar air turbine at 800 ºC
- combine solar power with state-of-the-art turbines to create steam up to 590 ºC
- run the highly efficient supercritical carbon dioxide Brayton cycle up to 700 ºC.
Pilot-scale research facilities
Recently, through ASTRI and the Australian Renewable Energy Agency (ARENA), CSIRO has been developing three high-temperature pilot-scale research facilities on the solar towers.
Tower 2 already has the Falling Particle Technologies installation, which is a system that uses solid media as the heat transfer fluid and thermal storage media. This system stores solar energy as heat up to 800 ºC. We are also building a high-temperature Integration Test Facility at Tower 2 using liquid sodium metal as the heat transfer fluid for temperatures up to 740 ºC.
On Tower 1, we are developing an ARENA-funded beam down reflector system for a pilot-scale 250kW thermochemical hydrogen reactor in collaboration with Niigata University, Japan. Initially, this facility will be used to generate hydrogen via the redox process, using ceria powder at temperatures of 1,100 to 1,400 ºC. This is a potential solar fuel pathway to generate hydrogen to export to Japan, competitive with hydrogen generation via electrolysis.
Supercritical steam
We are using solar energy to generate hot and pressurised ‘supercritical’ steam, at the highest temperatures in the world, outside of fossil fuel sources.
Supercritical solar steam is water pressurised to high pressure and heated using solar radiation. CSIRO achieved a world record in 2013 with the highest temperature supercritical steam produced from solar energy, at 591 ºC at 23.1 MPa.
[Music plays and text appears: Supercritical solar steam: the new frontier for power generation]
[Image changes to show an array of mirrors reflecting sunlight onto a solar tower and then moves to show moving solar panels]
[Image changes to show Mike Collins, Research Projects Officer, CSIRO Energy Technology]
Mike Collins: Solar thermal energy works by concentrating sunlight using mirrors. The light is then shone up on top of the tower where there’s a solar receiver and in that receiver there’s a panel of tubes which steam is flowing inside. That steam is heated to high temperatures and then it flows back down the tower to a turbine at the bottom of the tower, a steam turbine. The steam flowing through that turbine spins the generator to generate electricity.
[Image changes to show Robbie McNaughton, Research Projects Officer, CSIRO Energy Technology]
Robbie McNaughton: The temperatures that we’ve obtained are over 550 degrees and at pressures above 24 mega Pascals. This is called supercritical steam generation and it’s a state where steam actually transforms without boiling.
[Camera moves back to the solar panels and solar boiler]
The steam conditions that we’ve achieved are comparable to what is running at the moment in fossil fuel power stations. So we’re able to actually either displace the steam that goes into these, reducing the fossil fuel reliance, or in some cases maybe even replace fossil fuel completely.
[Image has changed back to Robbie]
It’s really exciting to work on these types of projects. Doing a world first is always exciting but in this case what we’ve actually been able to do is potentially make a step change in the way solar thermal power is generated.
[Music plays and CSIRO logo appears with text: Big ideas start here www.csiro.au]
Indirect solar cylinders
Frequently in winter the balance of energy required to satisfy the demand is supplied by the primary heating appliance – a heating boiler or direct-fired water heater. During the summer period, the solar energy absorbed by the collectors and transferred into the hot water can negate the need for any energy at all being provided by the primary heating appliance – this can have a significant impact on reducing carbon dioxide emissions and reducing energy bills.
For example, consider the system in Figure 4, a direct-fired water heater system to raise the incoming cold water supply at 10°C to a legionella-safe water temperature of 60°C (ie a separate boiler is being used for the space heating). A pre-heat cylinder served by an array of roof-mounted solar collectors can be used to supply pre-heated feed water to the directfired storage water heater, so requiring less fuel to raise the water to the required set point of 60°C.
Figure 4: Schematic of an example solar thermal system integrated with a direct gas-fired water heater
In the summer months there may be sufficient solar irradiation over prolonged periods of the day, such that the water in the preheat cylinder is able to reach temperatures in the region of 75°C to 80°C. In such circumstances, depending on how the pre-heat cylinder and collector array have been selected, the solar energy could be sufficient to supply the required water temperature at the outlets. At these water outlet temperatures it would be necessary to install a thermostatic mixing valve between the hot outlet of the pre-heat cylinder and the storage water heater, unless there are point-of-use mixing valves used within the building.
Another example is where commercial boilers are being used for space heating and the generation of hot water via an indirect calorifier. For boiler applications, the main principle of generating the solar thermal energy and the use of an indirect cylinder are the same as for direct-fired water heaters.
The difference is in the design of the cylinder, in that it frequently would have two indirect coils (although some systems do use a separate pre-heat cylinder) as in Figure 5.
At times when there is insufficient solar energy to heat the water to the set point (eg 60°C) the commercial boilers would provide the additional energy required to raise the water temperature to the required set-point.
Figure 5: domestic hot water cylinder with two indirect coils (Solar heating design and installation guide, CIBSE 2007)
The control of the transfer of energy from the collector array and the indirect cylinder is conducted in the same manner, whether the primary heating appliance is a direct-fired water heater or a commercial boiler. There is a differential temperature control via a sensor at the outlet of the solar collector array and a sensor located in the lower portion of the cylinder. When the temperature differential is greater than, typically, 7K the control unit switches on the pump, allowing the energy captured within the solar collectors to be circulated and transferred into the water via the indirect cylinder coil. When the temperature differential is typically less than 3K, the pump is switched off.
For commercial applications, the issue of development of legionella bacteria in the solar cylinder is often the subject of much concern as the water could be stored at temperatures at which the bacteria can develop (20°C to around 45°C favour growth). This can be overcome with appropriate design and properly informed operation. For example, cylinder pasteurisation can be conducted through the use of shunt connection between the storage water heater and the pre-heat cylinder and, for twin coil cylinders, a destratification pump can be used.
Cylinder selection and design
There are a number of issues to consider when selecting the capacity of the solar cylinder and the design of the indirect coil. The issues apply to both direct-fired water heater and commercial boiler applications.
In order to maximise the SF, the capacity of the solar cylinder should be matched to the daily hot water demand. In so doing, during the summer months when the available solar irradiation is at a maximum, it may be possible to satisfy the whole hot water load, depending on the demand profile of the property. This could entirely offset the need to burn fossil fuel in the primary appliance to generate hot water.
For glazed flat plate collectors, the rule-of-thumb measure is 50 litres of stored water per m² of solar collector array (the active area rather than the gross physical area). Evacuated tubes are more efficient than glazed flat plates, particularly during the colder winter months, as this type of solar collector is less prone to heat loss via convection. For a given surface area the evacuated tube collectors can hold more than twice as much heat transfer fluid. As a consequence, the rule of thumb on sizing and selection is 70 litres of stored water per m².
This ratio between collector array surface area and the volume of stored water is one of the key factors in delivering the optimum SF but, at the same time, ensuring the collectors do not enter into frequent and prolonged periods of stagnation. Even if the correct volume of storage is selected for a given solar collector array, there is still the need to be able to discharge this useful energy into the water stored within the indirect cylinder. The relationship between the surface area of the indirect coil and the absorber surface area of the collector array is critical with regard to dissipation of the solar energy into the water. Very much like a burner, heat exchanger and water flow through a boiler, the solar collector array is the heat generator. If the heat is not discharged into the water, then the heat transfer fluid will return to the collector array and this will affect collector performance and could result in collector stagnation.
Conclusion
The application of solar thermal systems employs traditional skills with readily available technology. On new-build projects there is activity on the roof of the building, during which the solar collectors may be installed. Beyond the collectors, and in the plant room, the work is predominantly hydraulic and can be carried out by the mechanical contractor installing the remainder of the equipment. The presence of the solar collectors on the roof also offers a visible indication that the enduser has taken positive action and invested in a technology that reduces the carbon footprint of the building.
Solar thermal solutions offer an opportunity to significantly reduce carbon dioxide emissions – approximately 100 kg CO2/m² of collector array per annum when compared with a natural gas primary heating appliance (based on a gross thermal efficiency of 80% with gas 0.193 kg CO2/kWh) – contributing to the renewable energy commitment required by local authorities and the increased requirement to include LZC technologies. The impact is particularly marked as domestic hot water becomes an increasingly dominant load as building air tightness and insulation levels improve and space heating loads reduce.
© Compiled by Yan Evans, technical director at Baxi Commercial Division, and Professor Tim Dwyer, London South Bank University
Shading Considerations for Solar Thermal Collectors
The ideal location for a solar thermal collector array is one in which the sun’s radiation falls full and unobstructed on the collector surface between the hours of 9:00 a.m. and 3:00 p.m., with the collector facing the noon sun directly. However, the perfect location isn’t always available, so here are some pointers to help you decide whether a site is acceptable or not.
The roof is usually the best site for collectors. It’s higher up, so there’s less chance of obstructions to block sunlight. It is space that you’re not likely using for anything else and, unlike a ground mounting, is safe from most aggressors (vandals, large animals and small children included). It requires less piping between the collectors and the rest of the system.
While all of these are significant advantages, roof mounts are not always ideal. Your home may have beautiful or rare trees that shade the south-facing roof and keep your yard and home cool in the hot summer months. Those trees may also be home to wildlife that you do not wish to disturb or may help form a visual or sound barrier between you and your neighbors for privacy. In these cases and many others, a ground-mounted array or a shed-mounted setup in a sunny spot can be a better option.
No South-Facing Roof
Many collectors will work even if they’re not mounted on a south-facing roof. or even on a roof at all. You can set the collectors on a rack on an east- or west-facing roof space using saw-tooth mounts to keep them facing the sun. An awning or a porch roof can provide the space needed for a collector. They can even be ground-mounted.
The important thing is to ensure an open solar window. wherever you end up locating the collector array. As long as the sun heats it and your chosen system allows the siting, the only problem is the practicality of getting the collectors safely in place and facing the right way.
Partial Shade
Unlike PV arrays, collectors still work in partial shade – they’re just less efficient than in full sunlight. If your collector site has less than 10% shade throughout the prime collection hours, it’s still acceptable in most cases.
You may also be able to reduce shading. If the obstruction is something natural, such as a tree’s branches (which can block 50%-75% of all the sunlight passing through its leaves), you can cut back the offending parts to clear a path for solar radiation. Just remember that trees grow and solar collectors don’t, so you’ll have to maintain a clear path through the years or suffer reduced efficiency.
What’s the difference?
Solar thermal, or solar hot water, collectors absorb heat from the sun and transfer it to water or glycol to provide space heating or domestic hot water. The two most common types are flat-plate collectors and evacuated tubes, flat-plate collectors being the most dominant. They generally consist of a 4×8-foot glass-encased panel that contains a thin metal sheet, with a dark coating to absorb energy.
Beneath the sheet are coils filled with heat-transfer fluid. Fluid circulates through the tubing, absorbing heat and then transferring it to a storage tank. A typical residential system used to supplement domestic water heating includes two panels.
An evacuated tube collector contains several rows of glass tubes connected to a header pipe. Each tube is a vacuum, which acts like a sealed thermos and eliminates heat loss through convection and radiation. Because of this, evacuated tube collectors lose less heat to the environment than flat-plate collectors.
A small copper pipe filled with fluid (glycol, water, or some other antifreeze) runs through the center of the glass tube. The fluid heats up, vaporizes, rises into the header pipe, and transfers heat (through a heat exchanger) to another pipe filled with fluid. This fluid carries heat to the water tank. From there, water can be used for hydronic heating and domestic hot water or converted for other uses.
Solar Power
Solar photovoltaic (PV) panels convert sunlight into electricity. They have a silicon sheet made up of semiconductors. When light strikes the sheet, part of the energy is transferred to the semiconductors, which knocks electrons loose and allows them to flow freely through connected wires. This flow of electrons is called direct current (or DC). The current then flows into an inverter, which changes it into AC (alternating current), the power used by your appliances. If there is no immediate demand in your home, this power can be stored in a battery or returned to the electric grid.
Alaska is a rather unique place for solar energy because of the excessive summer sun and the virtual darkness in winter months, which means a few months a year where solar doesn’t contribute much. For example, the 12-kilowatt photovoltaic array at the Cold Climate Housing Research Center in Fairbanks produces more than 10,000 kWh from March-September (about 30 percent of the building’s electric demand) but only 1,833 kWh during the rest of the year.
Most residential solar thermal systems are used to offset primary heating sources. If you want to use solar thermal as a main heating source, you need some type of seasonal thermal storage system to bridge winter months. PV systems, in most cases, simply offsets electricity purchased from the grid.
With PV, you can produce more power from your panels year-round if you keep them free of snow and change the tilt angle twice a year. The most productive months for CCHRC’s panels are April and May, when they enjoy long daylight hours and also capture reflected solar gain off the snow cover.
Different types of solar thermal panels perform better at different times of the year. For instance, evacuated tube collectors produce more BTUs during the spring and fall shoulder seasons, while flat plate collectors produce more heat during the summer.
Which ones are better to install?
To figure out which is a better investment, you need to consider what your installation costs would be and compare them to your current heating and electricity prices. Let’s look at an example Fairbanks house.
A 1,000-watt PV array will produce about 1,000 kWh a year in the Interior. Assuming electricity of 20 cents a kilowatt-hour, that would save you 210 a year. A two-panel solar thermal system could produce roughly 7 million BTUs a year, offsetting either 54 gallons of oil (saving ~150 at 5000.50 a gallon) or 2,050 kWh of electricity (saving 410). In other words, homeowners with electric water heaters stand to save more from solar thermal than those heating with other fuel types.
The actual cost of solar thermal in Interior Alaska (roughly 5000.50 per watt over the lifetime of the system) is lower than solar photovoltaic (less than 4 per watt as of June 2019). Yet PV panels are still more common in Fairbanks largely because they are easier to install and retrofit, don’t require plumbing, don’t have to be integrated into existing mechanical systems, and have no moving parts (whereas solar thermal systems have fluid and pumps that must be replaced).
Solar thermal systems will also shut off once your hot water tank is hot, whereas solar electric will power the electric hot water heater up to 8 months of the year, and will continue to produce energy to offset other electrical loads even when the hot water tank is hot, thus always providing peak efficiency.
The actual output and cost of your system will depend on many factors, like the solar exposure of your particular site, the type of heating or hot water system, the type and number of heat exchangers required, and others. With the cost of solar constantly dropping and fossil fuels always in flux, solar is becoming an increasingly attractive long-term investment. Anyone with good solar access may be wise to consider these systems as an option.
Solar at CCHRC
We have two types of solar installations at CCHRC. Our solar photovoltaic system consists of a 10-kilowatt array that is tied to the grid to offset our electric use. The system provides the equivalent of approximately 15% of our power over the course of the year and paid for itself in approximately 9 years.
Solar data can be seen here: Check current solar data at CCHRC here.
An economic analysis of the system can be found below.
We also have a solar hot water collectors that provide heat to the building and charge a tank of water so we can store heat when it’s not immediately needed. Read more about it on our Thermal Storage page.