Water Abundance XPRIZE – Do the Numbers Add Up?

On October 22, 2018 a US$1.75 million prize was awarded to two companies for a way of providing abundant water at a price of no more than $.02 per liter using renewable energy.

The technology developed by the Skysource / Skywater Alliance condenses humidity from the air using electrically powered compressors. It’s basically the same process as in a domestic air conditioner unit that has water dripping out of it, except that the Skywater units will filter and then sterilize the water using ozone. This is an energy intensive process.

There are other processes for generating fresh water from abundant sea water that also have a reputation for consuming a lot of energy. Desalination is used by many coastal cities and regions to top up insufficient ground water supplies. About of half of Israel’s water supply comes from Reverse Osmosis (RO) plants that desalinate sea water from the Mediterranean. Desalination plants also provide about 30% of Singapore’s water supply.

Reverse Osmosis consumes about 3 kWh of electrical energy per 1000 liter (1 m3) of fresh water extracted. If produced from fossil energy sources such as coal, oil or natural gas this energy demand will result in CO2 output, contributing to global warming. If produced from renewable energy, it requires considerable investments in generating capacity on top of the desalination plants themselves.

How does the Skywater process compare to RO with regards to energy consumption? The Skywater website is not exactly helpful, as it present gibberish instead of actual data:

What are the power requirements for the machine?
The Skywater® 300 runs on approximately 7 -10 kilowatts per hour. It operates on 50hz or 60hz and either 208-240V (single phase) or 380-440V (3-phase). This power can be supplied directly or from a generator for portability.

The Skywater 300 is a unit that can generate up to 1100 l of water per day. The above quote was neither written nor checked by an engineer. Note that energy is measured in kilowatt hours (kWh) while power is measured in kilowatts (kW). There is no such unit in physics as “kilowatts per hour”. Whoever uses this term basically doesn’t know what they are talking about! A device drawing one kilowatt of power will consume one kilowatt hour of energy for every hour of use.

Let’s assume they meant a power demand of 7-10 kW (which is the same as 7-10 kWh per hour). That means a daily consumption of 168-240 kWh of electricity. With an output of up to 1100 l, this amounts to at least 150-220 kWh per 1000 l (1 m3). This is roughly 50-70 times more than the specific energy consumption of a Reverse Osmosis plant. Other commercial units of water generators have similar specs. For example the units offered by Water-Gen in Israel are quoted as consuming 310 kWh per 1000 l, or roughly 100 times the power consumption of reverse osmosis units.

Today we’re still a long way from having access such an abundance of cheap electricity from renewable sources that we can afford to use 50-100 times more of it than another proven solution would use. Installing solar panels or wind turbines to power RO plants is expensive and consumes land. Building 50-100 times more solar farms or wind turbines to generate the same amount of water using water-from-air technology instead would make little sense, at least within a reasonable distance of the coast where you could still pipe desalinated water from coastal RO plants.

Water-from-air technology may make sense only in limited areas such as mobile military units in remote areas where cost is no object (but only if humidity is not too low and it’s neither too hot or too cold, i.e. if they’re not deployed in a desert anyway).

On the present evidence, water-from-air technology is far from ecologically benign or economically viable, compared to more efficient technologies available. The first step would always have to be reduced use of conventional water supplies (e.g. better irrigation systems, growing less water intensive crops) encouraged by appropriate pricing and reuse of waste water for other purposes.