Lo-Tech Solutions: Solar Water Heaters and their Embodied Carbon

Dr. Harry Zvi Tabor was a British-Israeli physicist who is known for pioneering work in the field of solar energy in Israel. His work isn’t only relevant in the academic world, but had a big impact on the energy use of israeli cities. He developed a solar water heater that is the most common in the country, with 90% of dwellings getting their hot water from his technology (or similar) [1]. Locals know it as the “dud shemesh’’, and it’s the cornerstone of heating energy efficiency in Israel. Of course, other measures have been taken since the ’80s, when the heater was popularised.

The fact that makes this success story especially good is that it’s not based on high-tech research & development, which normally looks to turn expensive, cutting-edge solutions into economically viable alternatives. The solar heating landscape in Israel did, however, sprout from innovation.

90% of Israeli dwellings depend on solar heating devices for their hot water (Miriam Alster/FLASH90)

The ’70s oil crisis pushed an oil-dependent country (which has improved, but is still lagging behind in renewable energy) into a very common problem at the time, and they turned to a resource that is massively available in the country: solar energy. Many homes do have both electric heaters and solar heaters, in case the sun isn’t shining — which is rare for a city like Tel Aviv, with over 300 sunny days during the year, which translate in abundance of solar radiation. We can draw similarities between the current climate emergency and the ’70s oil crisis: both demand an urgent change in energy policy and high innovation in the energy matrix (albeit for different reasons: back then it was a massive surge in costs, now it’s due to rising surface temperatures). We can learn from the way Israel navigated the change when confronted with their black swan.

Solar heaters around the world typically consist of a collector (the flat part) that receives solar radiation and heats up water that circulates through it. The water is then stored in a tank that is coupled to the collector, where it stays warm for as long as the weather allows it to. The collector is not always present in the set up, as is common in less developed areas.

Basic functioning of a Solar Water Heater

This technology is economically feasible (with as little as a three-year payback time [2]). It also pays back the user very quickly when measured only by “embodied energy” — meaning energy used in setting it up is easily paid back in two or three years (this is a good measure as it is independent of energy price). But what is its real environmental cost? As we usually do at Dosta Tec, we resort to emissions through Life Cycle Assessment (LCA) to answer this question. It turns out, the “environmental payback time” is also extremely reasonable. In other words, the yearly energy savings provided by solar water heaters are enough to offset the carbon emissions incurred during mining, manufacturing, and installation — and this happens quite fast too.

As noted in a previous article of ours, embodied carbon is strongly dependent on both a country’s energy mix (the “cleaner” the grid’s sources are, the milder the positive impact a solar heater will have) and the availability of solar heaters in a particular location. The more they travel, the more carbon they embody. Even accounting for these variations (and uncertainties), solar heaters are an exceedingly good idea, way more often than not [3]. Their payback time is so small, that they’re a great choice even when accounting for the installation of back-up electric heaters. In Portugal, the price of a simple solar heater with an efficient heat collector can start at as little as 1000€, with a proven environmental payback time of less than three years [4].

When choosing technology (or making any engineering decision, really) we always compare alternatives. The most robust solutions always come from scanning the available options. In this sense, a study from the University of Liverpool [5] compared technologies that are usually used to increase energy performance in buildings. Solar Thermal (ST, outlined in green in the graph below), is significantly better than, for instance, Solar Photovoltaic. They achieved this by measuring the carbon embodied during the whole life cycle (typically using cradle-to-grave strategy) and dividing by a standardised unit of energy efficiency, known as a Functional Unit (FU). This might sound confusing, but just think of it like this: it costs so much carbon to build and install this technology, and we divide it by how much energy it saves during its lifetime. This way we can compare technologies in a cost/benefit manner. The graph speaks for itself.

Comparing building-efficiency technology by embodied carbon equivalent [5]. GSHP: ground source heat pump | ASHP: air source heat pump | ST: solar thermal | MVHR: mechanical ventilation and heat recovery | SPV: solar photovoltaic | LED: light-emitting diode (lighting)

So, why doesn’t everyone living where the sun shines use them? There are multiple barriers, on several fronts, to implement this technology: legislative, cultural, and organisational. Life cycle assessment studies and the concept of embodied carbon are relatively new to both policy and industry (in the context of the fossil fuel problem). And we sometimes tend to think all innovation must be at the forefront of new technology. Efficient, low-tech solutions often appear when necessity calls, such as an oil crisis, or just general lack of economic resources, as happens in many developing countries. As we keep pushing the boundaries of energy efficiency towards regeneration, we should remember that the energy problem is complex and shouldn’t expect to solve it with a single, high-tech silver bullet. Instead, why not source a portion of our knowledge and practices from simpler highly efficient options? We might just hit the right technology mix and move closer to a real, cost-efficient solution.

Mateo Barbero


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[1] https://www.reuters.com/article/idUS311612153620110318

[2] Soteris Kalogirou, “Thermal performance, economic and environmental life cycle analysis of thermosiphon solar water heaters”, Solar Energy, Volume 83, Issue 1, 2009

[3] Battisti, R., Corrado, A.: Environmental assessment of solar thermal collectors with integrated water storage, in Journal of Cleaner Production 13, p. 1295–1300, 2005

[4] Fernandes, V. et al., “Life cycle assessment of solar thermal systems”, AHS Sustainable Housing Construction 2014

[5] Finnegan S. et al, “The embodied CO2e of sustainable energy technologies used in buildings: A review article”, Energy and Buildings, Volume 181, 2018, Pages 50–61

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