In the early 1950s, two countries led the world in developing solar hot water systems (SHWS): Israel and Australia. Building on technology pioneered in California and Florida in the 20s, both countries developed ‘integrated tank’ models: water is heated by pipes in 2-3 sqm of dark copper plates behind glass (the absorber), then flows to and from a tank mounted horizontally above the absorber.

As water in the absorber is heated by the sun, it rises up the sloping panel into the tank, at the same time drawing down cold water into the bottom of the panel to be heated – a closed loop or thermosiphon. Simple, self-regulating, and easy to manufacture, it first seemed like an elegant solution; given the plentiful sunshine in both countries, it seemed a ‘no-brainer’ that this technology would be widely developed and implemented.

Fast forward 70 years: in Israel, hardly a building lacks some form of SHWS; in Australia, less than 5% of houses have one. The big question is: if both countries started out with the same enthusiasm, technology and access to sun, why did Australia fail?

The answer is usually believed to be twofold: cheap energy and a lack of incentives. Energy in Australia was cheap for ages, and an SHWS was expensive (relative to an electric or gas storage heater). Payback periods of 6 to 8 years were considered too long; it's a really stupid argument, since no one asks the payback period of a stone benchtop over laminate or a big window over a small one. Never mind that an SHWS will keep saving money for decades, while the bench and window won’t.

Another reason was a lack of government regulations or incentives to install an SHWS (as had been in Israel since the 70s). The few rebates eventually introduced were state or council-based, periodic and without mandatory installation requirements. No national co-ordination, confused information and poorly promoted (we need to add that over 70 years the conservative LNP has been in power for twice the time of the ALP).

But the real failure, rarely acknowledged, was design: an integrated tank-style SHWS is ugly, ungainly, and difficult to install. The elegant idea of the original design was more than offset by the eventual clunkiness in appearance and the clumsiness of installation: great theory, terrible practice.

No manufacturer made an effort to visually integrate an SHWS onto Australian roofs; rather, the tanks were finished in bright aluminium or polished stainless steel, not coloured to match the roof. And adding insult to injury, the manufacturers splashed garish signs across the entire tank; no other household product had this aggressive, not to say offensive, branding.

Add to this the impracticality of installation: north-facing roofs required just structural reinforcement, but any other direction needed extra structure AND a separate frame, uglier than the SHWS itself. The increasingly popular 2-storey houses also needed the SHWS craned in.

In any event, the plumber had to install multiple fixings to secure the panels and tank, plus several penetrations for the hot- and cold-water pipes and the booster connections. All this on our preferred terracotta or concrete tiles, getting more brittle with age, not to mention being on those fragile tiles for maintenance. Two generations of plumbers, the most expensive subcontractor on site, became inured to the attractions of the SHWS.

Some councils – like the then Leichhardt in Sydney – had a policy mandating an SHWS but prohibiting them from being visible from the street. This highlighted the contradiction facing homeowners: they wanted to be eco-conscious, but so difficult were the issues of aesthetics, efficiencies, approvals, frames and overshadowing that they preferred not to install a system.

But wait, there’s more: in cold or frost-prone conditions, the water can freeze and burst the absorber. The solutions, using a separate circuit of liquid with an antifreeze agent or pumping warm water through on cold nights, increased the initial price and created other problems of failure. Hailstorms, common in coastal Australian cities (more than in the Middle East), can wreck the panel’s laminated or tempered glass.

One of the most vexing problems was the boiler: using electricity or gas in cloudy or rainy conditions. The booster wouldn’t know when the sun was about to come out or when the users were about to use the water, so the system defaulted to using energy to heat the water overnight in case there was early morning demand for hot water. Early morning sunshine (or owners showering late) meant the solar input would be wasted in a pre-heated tank. A later solution was to use instantaneous gas to heat only the water that was not hot enough.

Early issues were eventually resolved, but the design never suited Australian houses and conditions, and the public resisted installing them, so mass production to reduce prices never happened. Even more amazing, the manufacturers stuck with this design for over 40 years, spending more time seeking government rebates than addressing the design failures.

Fortunately, two technological changes improved the situation. About 20 years ago, a better absorber using evacuated tubes became common; it worked on a wider range of roof angles and orientations. The panels were still vulnerable to hail, but they were more efficient and eliminated the need for a bulky frame. Equally important, the tank was now separate from the panels, located on the ground and connected by a pump, making it easier to install and maintain and allowing instantaneous gas boost to be connected.

More recently, there has been an even more radical change. Hot water can now be produced by a heat pump motor. Like an AC set to heat, they are super-efficient: every kWh of energy input ‘pumps out’ 4 kWh of heat (a coefficient of performance, or COP, of 4). Now if we add a PV panel system, then the energy for the heat pump, and thus the hot water, comes for free.

Technically, this is a ‘solar water heater’ but more efficient than the traditional types, particularly if the heat pump is set to run when the sun is out and the PVs are working, and the backup has a COP of 4 when it’s not. The installation is easier: the tank and pump are on the ground near the kitchen/bathrooms, and the PVs are easier to fix and more hail-resistant than the older ‘hydronic glass panels’.

Despite 70 years of innovation, water heating still produces as much greenhouse gas in Australian houses as air conditioning. Fifteen years ago, when I first wrote some of this critique in Architectural Review Australia, I called for more SHWS, saying:

“Australia needs a coherent program to promote solar water heating for 95% of its dwellings, particularly in widespread suburbia that has access to sunlight on almost every roof. Ten years from now, travellers arriving by plane should remark on three things: the red terracotta roofs on the houses, the blue swimming pools and the black solar water heaters that are offsetting the energy demands of the former two.”

The sentiment hasn’t changed, just the technology to deliver it has: the ‘black’ that I so ardently wanted is now PV panels on 2.2 million homes; it needs to be linked to heat pump hot water systems, storing energy in hot water, and then the last old-style SHWS can be taken down and consigned to the Powerhouse Museum.

Cross-Sections are Friday musings from Tone Wheeler, formerly of Tone on Tuesday. Amateurs draw plans; professionals draw sections – never at cross purposes.

The slow, unlamented demise of solar thermal water heating

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