Can New Solar Innovations Save Us From The Consequences Of Dangerous Heat Waves?

by Risto Isomäki

Extreme heat waves have become much more common due to global warming. Urbanization is making the problem worse. One half of humanity is now living in urban areas and in large cities the urban heat island effect can increase temperatures by up to 12 degrees centigrade, on top of global warming.  According to some predictions the human mortality caused by heat waves could increase 50-fold or more during this century due to our planetary overheating problem.

Air conditioning is already responsible for 10 percent of all anthropogenic greenhouse gas emissions, when we take into account both the power produced for air conditioning devices and the coolant losses. We urgently need more climate-friendly ways to keep our buildings cool. Can new solar innovations solve the problem?

During 1951-1980 heat waves covered, at any given time, on average 0.1 – 0.2 per cent of the Earth’s surface. During 1981-2010, the figure was 10 per cent. In other words, the probability of a major heat wave affecting a certain area seemed to have increased by fifty or one hundred times. After the great heat wave in Europe during the summer 2019 statistical analysis produced roughly similar statistical conclusions. 

It is sobering to remember that this has happened because the Earth has heated by roughly one degree Celsius, only. What if our home planet will heat another degree, or two, four or five more degrees?

In Europe it seems that after a certain threshold temperature, each extra centigrade of heat during a major heat wave will increase human mortality by roughly 3 per cent. For example the great European heat wave in 2003 caused approximately 70,000 premature deaths, but the situation could soon become much worse. The temperatures in the Moscow heat wave in 2010 exceeded 40 degrees Celsius and according to a recent computer simulation an otherwise exactly similar weather event later during this century could produce 8.4 degrees higher temperatures. According to some estimates the mortality caused by heat waves could increase further -- by a factor of 50 or more -- before the end of this century.

When do we reach lethal combinations of heat and humidity?

This far the people who have died during heat waves have mostly been elderly or sick people who have died somewhat prematurely because of various chronic diseases or the general fragility caused by old age, making them vulnerable to heat. However, if the world heats by a few more degrees, we could soon see heat waves that are lethal to all people who will not be able to escape the heat inside air-conditioned buildings, including young and healthy people at their prime age.

We can survive even 50 or 60 degrees Celsius when the air is dry, because we can then regulate our body temperature effectively by sweating. Each gram of sweat that evaporates from our skin removes 2,260 joules of heat. However, when the relative humidity of air exceeds 90 per cent sweat no longer evaporates but just drops off our skin.

For this reason a much lower temperature combined with a very high relative humidity is lethal for us. We will die in six hours if we are exposed to a temperature of 35 degrees Celsius and a humidity of 100 per cent, or a temperature of 40 degrees Celsius and a humidity of 75 per cent.

This kind of conditions have never yet been seen on Earth by humans, but we are getting uncomfortably close to the lethal range. In July 2015, in Bandar Mahshahr in Iran, the conditions approached, for the first time in human history, a lethal wet-bulb temperature. The relative humidity was 50 per cent and the temperature 46 degrees Celsius, which means that our margin of safety is getting dangerously narrow. Many scientists now predict that we will see lethal wet-bulb temperatures during this century in much of the humid tropics and possibly also in many areas much farther from the equator where large irrigation programs could raise the relative humidity of the air to dangerous levels.

The inadequacy of the present air conditioning solutions

According to current estimates air conditioning is already responsible for approximately 10 per cent of all anthropogenic greenhouse gas emissions.  Most of these emissions are caused by the fossil fuels used to produce electricity for air conditioning and dehumidifying. The rest of the emissions are coolant leaks.

At the moment the state of the art in air conditioning devices is a substance known as R32 (or HFC 32).  It is a great improvement from the CFCs which were still widely used in the 1980’s, some of which have global warming potentials (GWPs) amounting to 15,000 – 20,000, meaning that each kilogram that leaked into the atmosphere was heating the planet as effectively as 15 or 20 tons of carbon dioxide. CFCs were also depleting the stratospheric ozone layer, protecting us from the most energetic and dangerous forms of ultraviolet radiation.

R32 transfers heat or cold so efficiently that only very small amounts of it are needed, approximately one kilogram for a household-sized air heat pump or air-conditioning device. However, its GWP is still relatively high: 625, meaning that if the one kilogram of coolant from an air heat pump will be released as a gas in the atmosphere it will heat the globe as much as 625 kilograms of carbon dioxide or 170 kilograms of carbon. There are coolants with a hundred times smaller GWP, but they cannot transfer heat or cold as efficiently. Because R32 has enabled the manufacturers to increase the SCOP or the seasonal electricity in-heat out ratio of air heat pumps from 1.5 – 2 to 4 - 6 and the similar ratio related to cooling to more than 10, it is currently the best compromise we have.

We need something still better, because the need for air conditioning is about to explode during the next half a century.  The purchasing power of most of the people living in hot or very hot conditions is increasing rapidly. This means that we could, after fifty years or so, have a 5 – 10 times larger number of households using air conditioning devices and electric air dehumidifiers, and the average size of all these apartments could be much larger than today.  If the world will heat up by 2, 4 or 5 degrees Celsius during the next one hundred years, the need for air conditioning will increase even further. Also the urban heat island effect is likely make the problem worse. Large cities can be up to 12 degrees Celsius warmer than the surrounding areas, and the problem could escalate if most of the world’s population will soon live in megalopolises, huge conglomerations of cities grown together.

In the hot and moist parts of the USA, installing a 600-watt air humidifier in every room has become popular among upper and middle class households. 

In other words: even if we met all our future air conditioning needs with the best currently existing electric air conditioning and dehumidifying technologies, ten or perhaps even twenty billion electric air conditioning and air dehumidifying devices would still produce more than enough greenhouse gas emissions to push the world over the edge into a full-fledged climatic chaos. 

Above all: the air in moist tropical areas might soon become lethally hot and moist during the most extreme heat waves for the poor families who cannot afford to buy and use efficient air conditioning.  In lethal wet-bulb temperatures a mere fan cannot prevent the overheating of a human body.  

Fortunately, there might be affordable, solar-based solutions to these problems.

Solar energy innovations for cooling our houses

Between 2017 and 2018, the average price of solar power dropped by 14 per cent, during a single year. In 2018 a Bloomberg NEF report predicted that the cost of photovoltaic cells will drop further by at least two thirds by 2050. Even larger drops in the price may take place if silicon solar cells will be replaced by perowskite. Perowskite minerals are as abundant as silicon but they are much easier to process. According to many experts for example old newspaper printing presses could probably be retrofitted to produce perowskite solar cells.

This means that photovoltaic panels cooling water, bricks, sand or stones – or making ice – during the day and this way storing the cool for the night may soon become an important solution to our air conditioning problems.

Another cost-effective method is reflective insulation.

Reflective insulation means cooling the building by covering it with a highly reflecting surface, like shining white paint, which has traditionally been used for this purpose in dozens of different countries. When white paint is too expensive, anything that is available and affordable and makes the roofs and walls whiter, lighter or more reflective can be useful, like white sand, chalk or salt.  

This can reduce the problem significantly, by preventing the overheating of the structures of the building itself. However, when the outside air becomes too hot and moist, reflecting outer surfaces can no longer solve the whole problem, alone.

Therefore, it might be a good idea to equip houses with some kind of heat insulation, even in the tropics.

In the tropical and subtropical areas houses are almost never equipped with proper heat insulation. Heat insulation is typically only used in the more northern and colder climates against the freezing winter-time temperatures. In the tropics it has been thought that it does not make sense to insulate houses against heat because apartments are also heated by waste heat created by human bodies and all kinds of electric equipment, as well as by the hot outside air flowing through the houses which are, in most cases, not that air-tight.

However, it might be a good idea to re-evaluate these assumptions in the light of some major technological changes which have happened in many related fields during the last few decades.

Modern electric equipment no longer produces (very) large amounts of waste heat. Twenty or thirty years ago the electric equipment in a typical European household would have produced a few kilowatts of waste heat, and the same would have applied to upper class or higher middle class homes in the tropics. Adding heat insulation to tropical homes with electrical appliances producing kilowatts of waste heat would not have been a clever idea because it would have created oven-like conditions.

However, much has changed since then. A typical LED light only consumes between 2 and 6 watts of power, instead of 80 or 100 watts. Thanks to new super-heat insulators like silica aerogel a modern, A+++ combined fridge and freezer only consumes 70 or 80 watts of power instead of 1 – 2 kilowatts, and this is rapidly becoming the new standard. Induction cooktops wasting hardly any power are quickly replacing old-fashioned gas stoves and electric stoves that liberated 50 or 60 per cent of the electricity they used as waste heat, not to say anything about traditional biomass stoves and cookers that have an efficiency of 5 – 20 per cent only. Pressure cookers are saving a lot of energy. LED televisions consume only tiny amounts of electric power. And so on.

This means that the average waste heat load from electric equipment of a middle or upper class household has become much smaller than it used to be and is rapidly diminishing further.  

The body of an adult human being only produces around 80 watts of heat when the person is resting. This can rise to more than 1,000 watts in heavy exercise, but people do not exercise much when they are at home, especially during major heat waves. This means that the bodies of a family of six only produces something like 500 watts of heat, and the average waste heat load produced by electric equipment in upper middle-class homes probably isn’t much higher than this, not to say anything about the lower middle class and poor households.

Also, even if all the air inside an apartment would be changed once in a few hours this would not heat the house effectively, because the heat capacity of air per cubic meter is 4,200 times less than that of water.  When all the air inside a 25-square-meter house with a height of two meters is replaced by outside air that is 20 degrees centigrade warmer than the air inside the house, the amount of heat transported inside the house via this exchange only amounts to 1,000,000 joules (= 1,000 grams per cubic meter x 50 cubic meters x 1 joule/gram/K or C). If all the air inside the house would be exchanged once every three hours – which is probably significantly more than the real average – the replacement of cooler inside air with hotter outside air house would only heat the house with the average power of 0.1 kilowatt (one kilowatt-hour = 3,600,000 joules). Which is, once again, rather negligible.

However, if our 25-square-meter house would have 40 square meters of non-heat insulated concrete, brick or metal plate roof and 150 square meters of non-heat insulated, South-, East- and West-facing concrete walls heating to 50 – 80 degrees Celsius in direct sunlight, a lot of heat will be conducted inside the house via the roof and walls.

Non-heat insulated concrete walls conduct heat very efficiently. The so called lambda or C (calorific) value of concrete is very high: 1.7. This means that a one-meter-thick block of concrete conducts heat with a power of 1.7 watts per square meter for each degree (Celsius or Kelvin) of temperature difference. In other words, when the concrete wall is only 10 centimeters and not 100 centimeters thick and the temperature difference between its outer surface and the air inside the house is 30 degrees (Celsius or Kelvin), each square meter of wall heats the inside of the house with a power of 500 watts, as much as the heat produced by the bodies of six adults resting inside the house.

This is only a very rough and simple back-of-the-envelope-type calculation. However, it might make sense to conduct a series of experiments in which different types of tropical and subtropical houses would be equipped with some cheap, affordable and simple heat insulation. 

Even a thin layer of any kind of heat insulation could make it much easier to keep an apartment cool with a single, small, solar-powered air conditioning machine, even during extreme heat waves. Finnish architects are already planning to initiate an experimental pilot project along these lines in Abidjan, in the Ivory Coast, but it would be important to conduct many different experiments with varying housing structures, designs and materials in many different climatic zones.

Reducing the relative humidity by moisture traps

Also the moisture traps based on salt that are already widely used for dehumidifying the air and for replacing the much more expensive electric dehumidifiers, especially in Southern European countries like Portugal, might constitute a partial solution to the problem.

Such moisture traps do not lower the temperature inside a home: they actually produce a tiny amount of extra heat. However, they can reduce the air’s relative humidity, which is an effective way of making hot days more tolerable or even comfortable, at least inside the houses.

Salt is a strongly hydrophilic, moisture-absorbing material.  Even normal table salt, which is mostly sodium chloride can absorb moisture from the air. It is very cheap and it can easily be dried in the sun and reused, over and over again.  However, sodium chloride can only reduce the relative humidity of the air to 74 per cent and it absorbs much less than its own weight of moisture. 

Calcium chloride is much more effective as a moisture trap: it can absorb several times its own weight of moisture before it has been converted to brine. Above all: it can reduce the relative humidity of the air to 50 per cent. According to the HUMIDEX index (measuring the combined impact of air humidity and temperature on people) reducing the relative humidity of air from 80 or 90 per cent to 50 per cent is often equivalent to reducing the temperature by 15 or 20 degrees Celsius.

When calcium chloride is dissolved in water, the reaction is exothermic and produces 700 joules of heat for each gram of salt. However, the same gram of calcium chloride removes 2.5 grams of water vapour from the air, which reduces the relative humidity of the air. If this once again makes it possible for the sweat to evaporate, each gram of sweat again removes 2,260 joules of heat from the human skin. Above all, this cooling impact is concentrated, in its entirety, on the surface of our bodies.

The main problems related to calcium chloride are that you have to be a little bit more careful when using it because it can irritate the eyes or even your skin, even though it is not really dangerous, and that a temperature of 250 degrees Celsius is needed for drying and reusing it.

The most cost-effective way to achieve the temperature of 250 degrees Celsius, which is required for the drying of the calcium chloride brine, is to use concentrated solar radiation. There are numerous already existing solar energy designs able to reach the required temperature range, some of which could be selected for this purpose. Calcium chloride is cheap, less than euro 100 per ton, but if it is not reused it can become a slightly harmful waste material damaging the plants growing in the soil.


Some further reading:

The frequency of heat waves may already have increased by 50- or 100-fold (and the rise of mortality per degree after a threshold temperature has been reached): Kenney, W. Carry; Craighead, Daniel H. and Alexander, Lacy M.: Heat Waves, Aging and Cardiovascular Health, Med Sci Sports Exerc 46 (10): 1891-1899, October 2014.

A similar heat wave in Russia later this century could produce 8,4 degrees higher temperature: Coghlan, Andy: Extreme heat for US and Russia, New Scientist, 24 March 2018.

Air-conditioning producing 10 per cent of anthropogenic greenhouse gas emissions and the current state of the art of the technology: Rajendran, Rajan: Refrigerants Update, Emerson Climate Technologies, Inc, September 19, 2011.

The wet-bulb temperature lethal for human beings:  Sherwood, Steven C. and Huber, Matthew: An Adaptability Limit to Climate Change Due to Heat Stress, PNAS 107 (21) 9552-9555, May 2010; Miller, Veronica S. and Bates, Graham B.: The Thermal Work Limit is a Simple Reliable Heat Index for the Protection of Workers in Thermally Stressful Environments, The Annals of Occupational Hygiene 51: 555-561, 2007: Muir, Hazel: Thermogeddon, New Scientist, 23 October, 2010; Pickrell, John: Too hot to handle?, New Scientist 20 January, 2018.

The urban heat island effect can exceed 12 degrees Celsius: Reducing Urban Heat Islands: Compendium of Strategies, Urban Heat Island Basics, Environmental Protection Agency, USA, 2014.

The characteristics of calcium chloride: Calcium Chloride: A Guide to physical properties, occidental Chemical Corporation, www.oxycalciumchhloride.com; Calcium Chloride: A Superior Choice over Silica Gel, Asborbtech, Weatherly Japan, K.K.

Expected drop in the price of photovoltaic power: Chivers, Tom: Rays of hope, New Scientist, 10 August, 2019