Solar Air-Conditioning: The Next Big Step for Solar Energy

Air-Conditioning (AC) has become an essential part of modern society as it enables a productive and comfortable life style in hot and humid climates. The amount of installed AC systems is expected to dramatically increase in the coming decades, largely driven by economic growth in developing countries. Since many of these countries, such as China, India, Indonesia and Brazil are in hot climates their AC use will be larger than in most Western countries [1]. Out of the 30 metropolitan areas with the highest demand for AC, all but two are in developing countries[2].

Air-conditioning is a very energy intensive activity. In the USA 5% of the electricity produced is used for AC, and up to 40% of the electricity demand for households in warm climates is due to AC. The rise of air-conditioning will cause a significant increase of fossil fuels.

It is difficult for a power grid to handle large amounts of air-conditioning. These power systems are characterizes by large peaks during summer days. In Israel, AC consumption requires 40% of the nation’s electricity during peaks while hardly any AC is required in the winter. Similarly, during peaks in Qatar 65% of the nation`s electricity goes to AC.  It will be very difficult for poorly developed grids, common in many developing economies, to handle the large increase of peak electricity associated with a rise of AC.

A good solution for decreasing the demand of fossil fuels and removing electricity peaks is to use thermal solar AC. This technology makes excellent use of solar power by directly utilizing solar heat to produce cooling and drying, instead of attempting to convert it to electricity. AC demands and solar power supply coincide forming a perfect combination of demand and availability. As opposed to PV solar cells, an AC using thermal solar power could harness up to 60% of the sun’s radiated energy.

Solar thermal AC can be categorized into three technologies: absorption systems, desiccant systems and vapor compression systems.

Absorption Cooling.

Absorption cooling was invented already in 1858 and is today a common technology. The energy input is usually natural gas or exhaust heat from cogeneration plants. Single effect absorption systems require a heat source of about 75° C and will have a COP of 0.7. Double effect absorption requires a warmer heat source at about 130°C but will also have a higher COP of 1.4. Absorption systems include low pressure chambers and are complex and expensive to construct. This, in combination with the cost of solar collectors, makes solar absorption expensive. It is nevertheless the most popular form of solar thermal AC today. Many traditional companies within the absorption cooling segment can today supply systems that work well will solar heat. There are also new companies, such as Climate Well, that have developed novel types of absorption technology specifically for solar use.

Vapor Compression

Conventional air conditioning uses a vapor compression cycle for cooling. Several new types of vapor compression systems have recently been developed which utilizes solar heat. Some of these systems rely primarily on electricity and merely uses solar energy to boost the AC. Other systems rely on solar heat to power the compressor. As opposed to solar absorption cooling, most of the companies supplying solar vapor compression are new and have developed technologies specifically designed for solar power.

Desiccate Cooling

In air-conditioning both cooling and dehumidification are important. Desiccant systems are a technology that chemically dehumidifies the air. Both solid and liquid desiccant systems have been used for solar AC purposes. An advantage of these systems is that they can utilize low grade heat at 50°C – 75°C. Several hybrid desiccant systems have been developed that incooperates either vapor compression or absorption technology.

Small incentives for Solar Cooling

There is a great demand for solar cooling and also several good technological solutions. In spite of this, there are very few installed systems. One of the main reasons for this is the lack of political incentives for this technology. In both the US and in Europe there are many effective incentives for solar power (solar energy used for electricity generation). Solar thermal technologies, such as solar thermal cooling and solar heating, do not however enjoy the same policies. Reports from the US[3] and Europe[4] have shown that the incentives for solar thermal energy are both to low and accompanied with complicated bureaucracy. Countries that have effective policies for solar thermal energy, such as Greece, Cyprus and Israel, are today world leading in solar energy use. Solar energy supplies 3% of Israel’s primary energy. This could be compared to the world leader in solar power – Germany, in which solar energy only supplies 1% of the country’s primary energy.

With the rapid increase in air-conditioning, solar cooling can be expected to become more important. It is unfortunate that this technology has not enjoyed the same rapid development as solar power, as it has the potential to both significantly decreases fossil fuels and stabilize electricity grids.

This article was first published in the “The Energy Collective”


[1]Lucas.W Dawis and Paul G Werler (2015) . Contribution of air conditioning adoption to future energy use under global warming. Proceeding of the National Academy of Science, Vol 112 no.19, 5962-5967

[2] Sivak, M. (2009). Potential energy demand for cooling in the 50 largest metropolitan areas of the world: Implications for developing countries. Energy Policy, 37(4), 1382–1384

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Top 3: Freaky Renewables

Renewable energy is more than just solar and wind. Here are three outliers among renewables that just might revolutionize power production.

Energy Tower


Up to 1000 meters tall and 450 meters  wide the Energy Tower is the monster of renewable energy. This technology combines  solar and wind by creating a downdraft inside a large tower. At the top of the giagantic tower water is sprayed into incoming hot dry air. This cools the air and causes it to drop, creating winds of up to 50 miles/hr. The air is diverted thru wind turbines and thus reliable, renewable energy is produced. The crux is that this technology only works if the tower very large, even a test facilitate would be several hundred meters high.

Most of the research on the Energy Tower has been done during the 80s and 90s at the Tecnion in Israel. By the early 2000s India and Israel investigated the feasibility of the project. Even though a long report points out the many advantages of the Energy Tower India and Israel decided not to proceed with the concept. There is one US company today that is planning on building a full scale the Energy Tower in San Luise, Arizona. According to this company the installation cost will be less than a third of solar power while the life expectancy is twice as long.

Salinity Gradient Power

The Norwegians do not worry about energy; they have a sea full of oil and a land full of hydropower.  They are in spite of this developing a new power technology that will be able to extract even more energy out of their salty sea and fresh rivers. Salinity Gradient Power is a technology that uses the mixing of salt water and fresh water to produce electricity. There is more energy in salt water and fresh water if they are separated compared to if they are mixed. People who work with desalination are bitterly aware of this as a large amount of energy is needed to extract potable water from sea water. Salinity Gradient Power is essentially reverse desalination; instead of spending energy extracting fresh water from salt water electricity is produces by mixing fresh and sea water. The theoretical maximum energy available from one cube of salt and fresh water is about 0.8 kwh – the same amount of energy available from one cub of water in a 280 m waterfall.

There are two main techniques for extracting power from salient gradient: Pressure retarded osmosis (PRO) and Ion Transport. In a PRO system a membrane separates the salt and fresh water. The fresh water will flow thru the membrane into the salt water, creating a high pressure. The energy from this higher pressure can drive a turbine to generate electricity. With Ion Transport technology electricity is generated by the exchange of electrons due to the difference in electrical potential between salt and fresh water.  Smaller demonstration plants built in Norway and the Netherlands have shown that Salinity Gradient Power does work. There are no full scale plants however, as this renewable energy source is not economical today.

Ocean Thermal Energy Conversion


Ocean Thermal Energy Conversion (OTEC) is a technology that utilizes the temperature difference in oceans between the warm surface water and the cold water at 1000 m depth to produce electricity. An Organic Rankine (ORC) cycle is used to transform the 25 ° C temperature difference to electricity. The ORC is similar to a conventional steam cycles (used in coal, gas and nuclear power plants) but uses a refrigerant instead of steam.  Because of the small temperature difference the ORC cycle will only have about 1-3 % efficiency.  For this reason a very large amount of water, pumped up thru a 1000 m long pipe, is needed to drive the cycle. The large and complex equipment, together with the harsh conditions at sea (waves, storms and corrosion), makes OTEC plants expensive to build. Many people believe that OTEC could still be a feasible power source as it generates cheap and predictable renewable energy. There are a handful of very small OTEC plants today, the largest one only supplies 100 kW. There are plans, however, on building commercial sized plants outside Hawaii and China.

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Why renewable energies are causing an increase of coal power in Germany

The German energy transition is the largest infrastructure project in the country since WWII. It aims to drastically transform the countries power production by adding large amounts of renewable energy while phasing out nuclear power.

Here are four key statistics from Germany´s energy transition.

1) Renewable energy has increased while nuclear power has decreased.

2) The amount of lignite and coal used has slightly increased since 2010. 


3) The carbon intensity of the energy sector has increased in Germany while it has decreased in Europe. The carbon intensity of the energy sector in Germany is 44% higher compared to France.

co2intensity europe

4) Electricity prices has drastically increased. (I wrote about this in the previous post “When welfare states fund the 1%”)

Federal Statistical Office (Statistisches Bundesamt) , Data on energy price trends

It is sometimes debated If Germany really has increased it’s use of coal and it’s CO2 emissions. The validity of these statements depend on which dataset timeframe that is examined. It is not debatable, however that the carbon intensity of the power sector has increased since 2010 and that the rapid increase of renewable energy in Germany has greatly increased the electricity prices.

In Germany, as in many other electricity markets, electricity is bought and sold either for the next day, or for the next hour. The price depends on supply and demand and there is also an option to import and export electricity. Renewable energies are guaranteed a price, and also the ability to sell all the electricity they produce. This creates a smaller market for fossil fuels. Because of the high marginal cost of gas power plants they cannot compete with coal power in the new smaller electricity market. Several gas power plants in Germany have closed the last years for this reason. This has created a situation where coal power has increased while gas power has decreased. The CO2 emissions from a gas power is about half compared to coal power, and therefore the transition from gas to coal has increased the carbon intensity of Germany’s power production.

This trend could continue as long as coal prices are low compared to gas. Since 2010 coal prices have slightly decreased while German gas prices have increased.

The price of coal and gas can of course change. Germany is currently subsidizing coal with 1.65 billion EUR but this subsidy is planned to be removed by 2018. This, together with a potential  CO2 tax or cap, could increase the price of coal making gas more competitive.

In summary; Germany is subsidizing both coal and renewable energy while closing its nuclear power plant. This has caused high electricity prices without a reduction in emissions.

Continue reading “Why renewable energies are causing an increase of coal power in Germany”

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Why bicycling to a BBQ is bad for the climate


My brother had a barbecue this summer in a lovely house 35 kilometers from Stockholm city. One of the guests, the new rabbi of the community, came all the way from the city on her bicycle. Needless to say, my family was impressed with this feat, and I really hope my brother gave the hungry climate hero an extra good piece of meat.  (Muslim and Jewish ritual slaughter has been banned in Sweden since WW II, so kosher meat is considered a special treat).

But I was left with a question:  how much better is it for the climate to bicycle instead of taking the car?

A car emits about 130gCO2 /km as it is burning fuel[1]. A small but significant amount of CO2 is also emitted in the production of the car[2].

Exactly like there are CO2 emissions associated with the proposal of a car, there are also CO2 emissions coming from the bike, or more exact from the biker. The energy input for the bicycle comes from the food that the bicyclist eats, and food is a very greenhouse gas intensive product. An average person in Sweden consumes about 1 200 000 kcal every year [3]and this contributes to about 2 ton of CO2 equivalent[4]. To make a 35 km bike ride an average adult needs about 850 kcal[5]. This will cause about 1400g of CO2 equivalent greenhouse gases to be emitted.

This means that a car ride emits about 3.4 times more greenhouse gases compared to a bicycle ride. So if four friends are going to a BBQ it is better, from a climate perspective, to be sharing a car instead of each one riding a bicycle.

Does this sound counterintuitive? There is a large focus in the media on greenhouse gas emissions from cars. Meanwhile, our food consumption contributes as much to climate change as our transportation[6] [7].  According to one study published in Nature, the global food system is responsible for a third for all greenhouse gas emissions[8].

Climate conscious consumers are asked to eat local and organic foods to decrease their CO2 footprints, but it is uncertain if this tactic is effective. Organic farming does emit, on average, less greenhouse gases compared to conventional farms[9], but they are also less land efficient[10]. The extra land needed for organic farming causes unnecessary deforestation. (Deforestation contributes to about a 10% – 20% of global greenhouse gas emissions  [11][12]).

Local food is also a tricky issue. In cold countries local food production is usually very energy intensive and therefore it could often be better, from a climate perspective, to import food from a warmer country[13].

kg co2e food
Numbers are taken from:

If one wishes to be climate conscious about food consumption there is really just one thing to do – to eat less meat. For example, one hamburger is equal to about 20 km of driving[14].  There are two reasons for the large climate footprint of meat. I) animals fart,  especially animals that ruminate. This causes a large release of methane, a very potent greenhouse gas[15].  II) An animal needs 10 kilos of feed to gain one kilo. This feed is often an agriculture product, such as soy or hay, and as such causes large greenhouse gas emissions.

In conclusion: if four friends are going to a BBQ they should take a car, but if they are going to a vegetarian restaurant it is better if they bicycle. 

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[2] About 96% of the CO2 emisions of a car comes from driving it.

[3] Average calories per day times days in a year time population of Sweden











[14] Assuming a 200 g hamburger.


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When welfare states fund the 1%

Tesla’s electric Roadster Sport

Imagine a country that loved luxuries cars, and the people who could afford them. It loved these cars so much that they were subsidized, and given tax exceptions. The country wanted to honor the rich people who drove these cars, and they were given access to special lanes and free parking.

Now, imagine a nearby country, where the government loved villas and the people who could afford them. They made sure that there were investment opportunities that only people with villas could take part in. The government even subsidized these investments by guaranteeing their returns.

If I told you that these were broadly left wing policies in socially conscious European states, it would sound preposterous, wouldn’t it?

However, these two examples mirror the realities in Norway and Germany, respectively, as their governments try to go green.

In Norway, the government has been heavily promoting the purchase of expensive electric cars. In an attempt to incentivize their citizens to purchasing them, all sorts of benefits have been created, including subsidies, tax exemptions, free parking and access to bus lanes. All of the bountiful bonuses are of course mainly enjoyed by the wealthiest in society.

Not surprisingly, Norway was the biggest market outside the US for Teslas luxury electrics vehicles, with starting prices at $ 112 000.

In Denmark, and more notably in Germany, private solar power is strongly subsidized. House-owners are encouraged to install solar panels on their rooves, and are guaranteed a high price for the electricity they produce. Many rich people living in villas with large rooves have used these benefits to great effect, widening the income inequality in the country as a result.

In addition to providing an incentivized revenue stream only for the wealthy, these incentives have created a high price for electricity; so in both Denmark and Germany household consumers pay much more for electricity compared with the rest of Europe.

This is perhaps not a problem for the rich who could afford investing in solar power or paying a high price for their electricity. It is a problem, however, for poor people who want to stay warm during the winter. This system, largely supported by the left, is taking money from the poor and giving it to the rich, in the name of environmentalism.

These two examples highlight the policy contradictions which often arises when left wing parties try adopt an environmentalist agenda, while simultaneously trying to narrow the growing wealth gaps in their societies.

Similar policies for reducing greenhouse gases, like high taxes on carbon and subsidies for private power production, hit the poor and benefit the rich. While this isn’t the place to advocate for one policy priority over another, I do propose that it is hard to have both, and that it’s an important discussion that is often swept under the carpet.,_second_half_2014_(%C2%B9)_(EUR_per_kWh)_YB15.png,_second_half_2014_(%C2%B9)_(EUR_per_kWh)_YB15.png

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Why the people in Chernobyl are lucky to have a nuclear power plant


A diorama showing the Chernobyl Nuclear Power Plant as it looked following the accident.
A diorama showing the Chernobyl Nuclear Power Plant as it looked following the accident

Many informed people, especially in the environmental movement, consider nuclear power to be dangerous. When asked why they will often give the one-word answer – Chernobyl. So how bad was Chernobyl, and how does nuclear power compare to other power sources?

The Chernobyl reactors were light water graphite reactors. Even though these reactors where unsafe, they were nevertheless used, as they had the ability to produce weapon grade plutonium. The Chernobyl reactors were as much a weapons factory as a power plant. Another feature that made the Chernobyl reactors unsafe was that they lacked shielding, something that was standard for nuclear power plants in the west at the  time[1].

When the Chernobyl power plant/plutonium factory exploded it took 10 days to extinguish the fire and the catastrophe lead to a major leak of radioactive gas. But how many people actually died in this terrible nuclear catastrophe? There are only 50 confirmed deaths from radiation from the accident.

It is very hard to estimate the total number of deaths from the radioactive fallout. One of the main reason reasons for this is the lack of scientific data, since there are so few recorded deaths from radioactivity. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) concludes that “There is no scientific evidence of increases in overall cancer incidence or mortality rates or in rates of non-malignant disorders that could be related to radiation exposure. The WHO has also made an attempt to estimate the number of deaths from the accident. Their conclusion differs from the UNSCEAR and their study showed that “A total of up to 4 000 people could eventually die of radiation exposure from the Chernobyl nuclear power plant”. The possible death of 4000 people is of course a terrible tragedy, but it is important to put it in perspective. How does this compare to coal power, the other popular power source at the time?

The health problems associated with coal power plants are so enormous that they are hard to phantom. The majority of deaths today from coal power are in India and China, where there is less strict regulation on emissions. (These regulations are actually good for fighting climate change, but that is a topic for a different discussion). In the US 13 00024 000 people die every year from Coal power and in the EU the number is about 18 000.

Most of these deaths can be attributed to air pollution, but many people also die when the coal is mined and in power plant accidents (not just nuclear power plants have accidents). An interesting measure of the safety of a power plant is average deaths per unit of power produced.  For every TWh  there is an average of 0.074 deaths from nuclear power while there are 77 deaths from coal power plants[1].

The Chernobyl nuclear power plant consisted of several reactors and some of them where active until 2001. During their life time they produced a total of about 220 TWh[2]. This means that if the power plant would have been a coal power plant instead of a nuclear power plant the population of Chernobyl could have expected around 17 000 deaths. So, are the people of Chernobyl happy that they got a nuclear power plant? Even with the Chernobyl catastrophe about four times more people could be expected to have died if the nuclear power plant had been a coal power plant instead[3]. What about radioactivity? Coal has even higher emissions of radioactive material compared to nuclear power[4].

One of the reasons for the fear of nuclear power accidents is that these accidents make headlines when other power plant accidents do not. Many people are aware about the Three Mile accident, Chernobyl and Fukushima. but who remembers the Banqiao Dam catastrophe in 1975? This hydroelectric power plant broke down and as a consequence 26 000 people died from flooding, 145 000 – 220 000 died from the subsequent famine and sicknesses . I am still waiting, however, for the day that the Banqiao Dam catastrophe will be used as an argument against hydroelectric power.

There might be good arguments against nuclear power but safety is not one of them, nuclear power is actually safer than most other power sources. In light of this it appears irrational that fear drives a country to close their nuclear power plants while keeping their coal power plant open. Then again, fear, especially fear for something complex or incomprehensible, often takes irrational forms.

[1] This refers to Chinese coal power plants. It is fair to assume that coal power plants in the Soviet polluted as much as coal power plants in China today.


[3] Of course the actual deaths from coal power plants are dependent on many factors such as hight of stacks and the surrounding geography


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Why the this amazing invention is useless

pedal desk cool thingscom
Picture from

Energy is hard to very hard grasp. We can look at a jug of milk and immediately know if it contains 1 or 10 liters. It is difficult to appreciate, however, how much energy is contained in one liter of gasoline. (The output of one man working hard for 10 days).

Pedal Power is an interesting company that can perhaps aid in the understanding of the proportions of energy. Their startup has developed a pedal desk, an amazing invention that enables a person to sit at their desk, and at the same time use their feet to pedal.

The energy from pedaling can then be used to produce electricity, for grinding coffee or for pumping water. Pedal Power has been cited in several large publications and has raised more than $30,000 from 400 investors through Kick Starter. It is easy to understand the appeal of this product for those of us who enjoy the thought of producing own bread or beer.  The developers explain in their promotion video how their product can “connect people to the energy they use,” and “reduce our footprint on the environment.”

It is the second point that makes the Pedal Desk so exciting, because it demonstrates how hard it is to appreciate the proportions of energy . Nobody, not the developers, not the investors and not the reporters writing about the company, has questioned the environmental footprint of the Pedal Desk. (Of course it is good for the environment, it is made by two cool dudes who like bicycling and it created  “homemade power”).

So I ask: what is the energy payoff of the Pedal Desk? Will someone using the desk be able to produce more energy than the energy required to build it?

In short –NO.  One would have to pedal for two hours every workday for 14 years, just to the make up the energy needed to produce the steel and to deliver the desk[1]. Added to this is then the energy needed to ship the steel and other materials, construct the desk and deliver the desk to the store. I would be shocked if a single Pedal Desk becomes a net energy producer. This also means that any CO2 savings from using the desk will be offset by the CO2 emissions from its production.

Pedal Power might be good for pedagogic purposes or in circumstances where there is no grid connection. It is not, however, an energy efficient method to power your computer, and it can definitely not “reduce our footprint on the environment” . Pedal Power is nothing more than an energy scam and, as such, an excellent example of how hard, but important, it is to appreciate proportions of energy.

[1] Someone who pedals every workday for 2 hours will produce 60 MJ/year, assuming an output of 70 W. To produce the 20 kg of steel needed for the desk, an input of about 400 MJ is needed, and almost all of this energy comes from fossil fuels. Let’s say that the average distance to a furniture  store with the desk in stock would be  roughly 50km away, so the person buying this desk would need to drive (a fairly big car) about 100 km to pick it up. This adds another 450 MJ to the energy input.It should be mentioned however that if the electricity produced by the desk was produced by fossil fuels about 65% of the energy in the fossil fuels would be lost.


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Why dirty coal power plants are good for the climate

The diagram below, taken from the IPCC 5 report, shows drivers of climate change and is essential for understanding global warming. It highlights the confusing fact that some anthropogenic (man-made) emissions heat the earth, by contributing to the greenhouse effect, while other emissions cool the earth by reflecting sunlight back into space.

radiative forcing

A great example of a confusing pollutant is coal power plants. These are often branded as the biggest criminal in climate court. But could they really be blamed for our planets fever ?

NASA conducted a study to evaluate the total radiative forcing of coal power plants, that is the total effect that coal has on the climate. The result suggest that “Emissions from coal-fired power plants until ∼1970, including roughly 1/3 of total anthropogenic CO2 emissions, likely contributed little net global mean climate forcing during that period”. The odd thing about coal power plants is that the “dirtier” the power plant is the less it will contribute to global warming. All coal power plants, dirty and clean, emit CO2. Even though CO2 contributes to the greenhouse effect, it is usually not considered a pollutant as it does not cause any local or regional environmental problems. Coal power also emits pollutants, such as sulfur dioxide, nitrogen oxides and particulate matter, which cause severe harm to local and regional air quality. Even though these pollutants are bad for the regional environment, they are good for the global climate as they cool the earth by blocking sunlight. Since the 1970s many western countries have enforced strict policies on pollution, and this has stopped coal power plants from emitting the pollutants that previously compensated for the greenhouse gas emissions. These policies have caused an increase in the total radiative force, thus contributing to global warming. The energy sector in China and India are at the same point today as the western countries were in the 1970s. The total contribution to global warming from these power plants is small today due to the pollutants that they emit together with CO2. A rapid increase of global warming could be expected when these countries introduce stricter pollutant controls.

Can we blame today’s global warming on China’s coal? No we can’t. These power plants are such a large source of pollution that they hardly contribute to global warming (in the short run). Even though an increased regulation would contribute to global warming, this should still be encouraged as pollution from coal causes more than a quarter of a million deaths every year in China.

The issue of regulating coal illustrates how environment policies are often both confusing and paradoxical. There really are no silver bullets when dealing with environmental issues as the solution for one problem often causes another.

Continue reading “Why dirty coal power plants are good for the climate”

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