Practice

Solar thermal

Also known as: Solar hot water, Solar heating

Direct use of sun's heat for hot water and space heating — distinct from solar PV (which converts sunlight to electricity). Solar thermal collectors absorb sun and transfer heat to water or air for domestic use. Practiced for millennia (Roman bathhouses oriented for sun); commercialized in the 19th century; mass-adopted in southern Europe and Israel since the 1970s. Solar thermal can supply 60-80% of household hot-water needs in moderate climates, often at lower cost and higher efficiency than equivalent solar-PV-plus-electric-water-heater systems.

Forms

Flat-plate collectors — black absorber plate under glass; circulates water or antifreeze; the most common residential form
Evacuated-tube collectors — vacuum-insulated tubes; higher efficiency, especially in cold climates
Batch / breadbox heaters — simplest passive form; tank inside an insulated glazed box; used for seasonal hot water
Drainback systems — fluid drains from collectors when not in use; freeze-protection without antifreeze
Combisystems — solar thermal for both domestic hot water and space heating
Concentrating solar thermal (CSP) — utility-scale; mirrors focus sunlight to heat fluid for steam turbines

Adoption

Israel — solar hot water on essentially every residential building; mandated since the 1980s
Greece, Cyprus, Portugal — substantial residential adoption
China — by far the largest installed solar-thermal capacity globally; ~70% of world total; widespread on apartment buildings
Germany, Austria — extensive central European adoption with combisystems
United States — modest residential adoption; commercial and pool-heating common

Why this matters

Solar thermal is often the lowest-cost solar technology for displacing fossil-fuel water heating. Particularly:

In sunny moderate climates — solar thermal payback is faster than [[solar-pv|solar PV]] for water heating
For high-hot-water-use households — large families, businesses with high hot-water demand
In mixed solar systems — combined with [[solar-pv|solar PV]], produces both electricity and heat from the same roof area

The technology is mature, low-tech (often DIY-buildable), and reliable for decades with minimal maintenance.

Sources

Solar Hot Water, Bob Ramlow and Benjamin Nusz (New Society, 2010)
IEA Solar Heating and Cooling Programme publications
Solar Energy Industries Association documentation

Rooted in life.

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