How does a solar water heater work?

Walk onto almost any terrace in Pune or Bengaluru and you'll see one: a fat white tank lying on its side, with a row of glass tubes sloping down from it towards the sun. That is an evacuated-tube solar water heater, the most common kind in India. A family-sized one holds 100 litres, enough for 3 or 4 people's baths.

Each tube is really two glass tubes, one inside the other, with the air sucked out from between them. The inner tube is coated dark blue-black to soak up sunlight. Water fills the inner tube and opens straight into the tank. Water warmed in the tube rises into the tank, and cooler water from the tank sinks down to take its place.

The tank is a stainless-steel drum wrapped in thick foam, like a giant flask. Cold water trickles in from the overhead tank, which sits higher so gravity does the pushing. Hot water leaves from the top and runs down to your bathroom. A thin open vent pipe lets steam and air escape. Many tanks also carry an electric backup heater for cloudy days.

The other kind is the flat-plate collector: a black metal sheet with copper pipes, in an insulated box under glass. Its tank must sit above it (chapter 4 shows why). Unlike an electric geyser (see WaterHeaterClear), nothing here uses a plug to make heat.

On a clear day, about 1,000 watts of sunlight falls on every square metre facing the sun at noon. That's like a small room heater on every square metre of your roof. A collector catches most when the light hits it square on. The more slanted the light, the more it spreads out and the less each square metre gets.

In India the sun climbs high and swings across the southern sky, so collectors face south. The best fixed tilt is about equal to your latitude: about 13° in Chennai, 18° in Pune, 29° in Delhi and 34° in Leh. Tilting a little more helps in winter, when the sun is lower and hot water matters most. Leh gets less sunshine on flat ground in January than Pune, but a steep collector there catches nearly as much.

Two tricks keep the heat once it arrives. The absorber has a selective coating: it soaks up about 95% of sunlight (high absorptance, α) but gives off very little infrared heat (low emittance, ε, around 0.05 instead of 0.9 for black paint). And the glass lets sunlight in but blocks the long-wave infrared the hot plate gives off. It's the greenhouse effect, the same reason a parked car gets so hot.

Once the water is warm, it starts losing heat to the cooler air, and the hotter it gets, the faster it loses it. Heat can escape in three ways (see HeatClear): conduction through the air touching the hot surface, convection as that warmed air rises and carries heat away, and radiation, invisible infrared light.

A bare black pipe loses heat all three ways and to the wind as well. On a breezy day it can barely warm water 10 °C above the air. A flat plate under glass traps a still layer of air, which slows convection a lot, and its coating cuts radiation. That's good for warm water, but not for hot.

An evacuated tube takes the air away completely. With no gas between the glass walls there is nothing to conduct or convect heat, so only a little radiation gets out, and the selective coating keeps even that small. It's the same idea as a vacuum flask. Crack the tube, let the air back in, and it loses heat about 8 times faster. You can spot a dead tube: the shiny silver getter patch at its bottom turns milky white.

Engineers sum this up in one line: useful heat = F_R·[τα·G − U_L·(T_in − T_air)]. τα·G is the sunlight soaked up, U_L is how leaky the collector is, and the bracket shows why efficiency falls as the water gets hotter than the air. No energy vanishes (see EnergyClear): every watt of sunlight either ends up in the water or leaks back out as heat.

Warm water is a little lighter than cold water: a litre at 50 °C weighs about 10 grams less than a litre at 25 °C. So water warmed in the collector floats upwards through the hot pipe into the tank, while heavier cool water from the bottom of the tank sinks down the cold pipe to take its place. The loop turns by itself as long as the sun shines. This is a thermosiphon.

The push is tiny: a few tens of pascals, the weight of a few millimetres of water. That's why the pipes are short and wide, with few bends. It's also why the tank must sit above the collector. Put it lower and the hot water has nowhere to rise to: the flow stops and the collector cooks. At night it's worse: the collector cools, its water gets heavy and sinks, and pulls hot water out of the tank to be cooled. That's reverse thermosiphoning.

Inside the tank the water sorts itself into layers, hottest on top (just like in a geyser, see WaterHeaterClear). The tap draws from the hot top, and the collector gets the coolest water from the bottom, which it heats most efficiently.

Big buildings use pumped systems instead: a small pump and a controller push the water round, so the tank can sit anywhere, even downstairs. It costs a little electricity and more parts to go wrong.

A solar water heater is sized in litres per day. The usual rule in India is about 100 litres for 3 or 4 people, since a bucket bath needs 15 to 25 litres of hot water. On a sunny day the tank climbs from lukewarm in the morning to 55 to 70 °C by late afternoon.

The water you use in the morning was heated the day before. Overnight the foam slows the heat loss, but the tank still cools by a few degrees. So the number that matters is the next-morning temperature.

How much the sun gives changes a lot. Pune in April is bright; Pune in July, under monsoon cloud, gets less than half as much. A Delhi winter has short days and cold water, so an electric backup heater often has to top the tank up before the morning baths. Even then, the sun does most of the work, and every unit of electricity it saves is about 0.73 kg of CO₂ not emitted by India's grid. It is also a unit that never has to travel to you over power lines and through transformers (see PowerLineClear and TransformerClear). The backup heater itself is a 2 kW load, so its circuit needs a proper MCB and RCCB (see MCBClear).

India has pushed solar water heaters for decades. The Ministry of New and Renewable Energy (MNRE) has run programmes since the 1980s, and many cities made them compulsory for new homes: Bengaluru (Karnataka, 2007, enforced by BESCOM from 2009), and Pune, Mumbai and other Maharashtra cities. Flat-plate collectors are made to IS 12933 from the BIS, and tube systems to IS 16542 and IS 16544. Look for the ISI mark.

Hard water is the big enemy. Heating makes the dissolved calcium come out as white scale. In a flat plate's narrow copper risers, scale chokes the thermosiphon flow; in tube systems it settles as sludge at the bottom of the tubes and tank. A yearly descaling and flushing by a technician keeps it working.

The monsoon cuts the sunshine in half in places like Pune and Mumbai, which is what the electric backup is for. Monkeys love sitting on warm glass tubes and hail can crack them, so many homes fit a wire mesh guard. A broken tube in a water-in-glass system lets the tank drain, so it needs fixing fast.

And safety: on a sunny day the tank can pass 70 °C, hot enough to scald in a second or two. Mix with cold water, and keep children away from the hot tap. Water also expands as it heats. That's why the tank has an open vent pipe (or, in pressurised systems, a pressure-relief valve): without one, a sealed tank could burst.