A solar thermal system converts sunlight directly into heat — usually for domestic hot water — using roof-mounted collectors, a circulation circuit and an insulated storage tank. Collectors convert 60–70% of the solar radiation striking them into usable heat, roughly three times the conversion efficiency of a photovoltaic panel, and a correctly sized system covers 50–70% of a household's annual hot water demand.
What are the main components of a solar thermal system?
Every solar thermal system is built from the same functional blocks: a collector that absorbs sunlight, a circuit that moves the heat, and a store that holds it until you open a tap. In a pumped (indirect) European installation those blocks look like this:
| Component | What it does |
|---|---|
| Solar collector (flat plate or evacuated tube) | Absorbs solar radiation and transfers it to the fluid |
| Heat transfer fluid | Carries heat from roof to tank; usually a glycol–water mix |
| Pump station | Circulates the fluid, shows pressure and flow, holds safety valves |
| Differential controller | Runs the pump only when the collector is hotter than the tank |
| Twin-coil cylinder | Stores the heat; solar coil low, backup coil high |
| Expansion vessel | Absorbs the fluid's volume change as it heats |
| Backup heater (boiler, heat pump or immersion) | Tops up on dark days |
The collector choice between flat-plate and evacuated tube designs changes performance at high temperatures, but not the basic architecture.
How does the heat travel from the roof to your tap?
Sunlight heats an absorber plate behind the collector glazing; a selective coating keeps the captured heat from radiating back out. When the controller sees the collector run typically 6–8 K hotter than the bottom of the tank, it starts the pump. The fluid picks up heat in the absorber, releases it through the lower coil of the cylinder, and returns to the roof. The tank heats from the bottom up, preserving stratification, and the backup heater only treats the top section — so every solar kilowatt-hour directly displaces purchased energy. When you open a tap, mains-pressure water leaves the top of the cylinder; the solar fluid itself never touches your drinking water in an indirect system.
How efficient is solar thermal compared with solar PV?
At domestic hot water temperatures a good collector converts 60–70% of incoming solar energy into heat, while a photovoltaic module converts around 20% into electricity. That gap is physics, not engineering maturity: heat is a lower-grade form of energy than electricity, so it is far easier to capture. The IEA Solar Heating & Cooling Programme reports more than 500 GWth of solar thermal capacity in operation worldwide, most of it doing exactly this job — heating water. The comparison with PV has more dimensions than raw efficiency, which is why we cover it separately in solar PV vs solar thermal.
What types of solar thermal system exist?
Two splits define the family tree. Pumped versus passive: pumped systems use a controller and circulator, while a thermosiphon system circulates by natural convection with the tank mounted above the collector — no pump, no electricity. Direct versus indirect: direct systems heat the tap water itself in the collector, indirect systems use a separate antifreeze circuit and heat exchanger; the trade-offs are covered in direct vs indirect solar water heating. Cold-winter climates almost always use indirect pumped or drain-back designs; frost-free coastal climates can use simple direct thermosiphons.
How much of your hot water will it actually cover?
A correctly sized system in central Europe delivers 50–70% of annual hot water demand — near-complete coverage in summer, a useful pre-heat in winter. Certified performance data matters here: collectors tested under Solar Keymark publish independently measured output figures, so coverage claims can be checked rather than taken on trust. Solimpeks has manufactured collectors in Konya since 2001, holds Solar Keymark certification across its Wunder collector range and SRCC OG-100 certification for selected Wunder ALS models; the seasonal split between summer and winter coverage is examined in detail in summer vs winter solar coverage.
Frequently asked questions
Does a solar thermal system work on cloudy days?
Yes, at reduced output. Collectors harvest diffuse as well as direct radiation, so overcast days still deliver a pre-heat rather than nothing. The backup heater covers the shortfall automatically, so hot water availability never depends on the weather.
Is solar thermal the same as solar panels?
No. Solar thermal collectors make heat; photovoltaic (PV) panels make electricity. They look similar on a roof but serve different loads. PVT hybrid panels combine both functions in one unit.
How long does a solar thermal system last?
Quality collectors routinely run 20–25 years or more — Solimpeks designs its collectors for a 20–25-year service life. The pump, controller and glycol fluid are the service items, replaced on normal maintenance intervals rather than with the system.
Do I need a special hot water tank for solar?
You need a cylinder with a dedicated solar coil in its lower section — usually a twin-coil model, with backup heating at the top. A standard single-coil cylinder can sometimes be replaced or supplemented when retrofitting.