One of the effects of rocketing energy prices is a growing interest in alternatives to existing natural gas imports. One solution lies in processes that come under the banner ‘power-to-gas’. The underlying idea involves the self-production of heat for homes through natural gas created synthetically. In summer, when there is plenty of sunshine and homes do not usually require heating, the excess power generated by photovoltaic systems can be used to produce methane from water and carbon dioxide. Methane is the main component of natural gas and can be used in this form without additional refining. The methane can therefore be fed into the existing gas network or held in the available gas storage containers. It can then be used to heat homes later on in the winter, when temperatures drop and very little solar power is available.
This not only keeps the home warm, but has the side-effect that any surplus electricity generated in summer can be used productively. Storing the surplus electricity in a battery is seldom cost-efficient, as it requires large and expensive batteries whose manufacture also carries heavy environmental costs. Converting excess electricity into synthetic gas therefore has the benefit that it can be stored for consumption later in a pre-existing repository, so no additional construction is necessary. In addition, such processes would further reduce dependence on gas imports and cushion the effects of market price volatility.
Efficiency is the sticking point
So the principle of power-to-gas for greater energy independence makes sense. To date, however, there has been one sticking point: efficiency. When electricity is converted into natural gas, much of the energy is lost as waste heat – up to half the energy used. And so far, only large-scale solutions have been available for using waste heat, for example in factories and waste incineration plants. This conundrum has now been solved by PSI scientists belonging to the research team of chemical engineer Emanuele Moioli: last year they presented a far more efficient power-to-gas system for domestic use. It uses the waste heat emitted to power the domestic water heating system. The system therefore not only provides fuel for heating the home in the winter, but supplies hot water for the entire year. “Hot water is still needed in the summer for activities such as showering, cooking and washing,” says Moioli.
The apparatus has two main components: a proton exchange membrane (PEM) electrolyser that employs electrolysis to produce pure hydrogen (H2) from water (H2O), and a small chemical reactor that generates methane (CH4) from the hydrogen and carbon dioxide (CO2). The process releases oxygen (O2) and waste heat. This heat is no longer wasted, however: the reactor itself acts as a heat exchanger and directly heats up the water to around 80 degrees for domestic use.
In a new study appearing in the journal RSC Advances, published by Britain’s Royal Society of Chemistry, Moioli has now shown that the apparatus can cover between 20 to 40 percent of the annual energy consumption of a large residential building, depending on its location in Switzerland. To do so, his computer model simulated the system’s use in a large residential structure comprising 64 households over 16 floors, equipped with solar panels on the roof and on the façade, with a total surface of 800 m2. He calculated the system’s performance in Brugg in the north of Switzerland, in Sion in the Alps and in Lugano in the very south of the country. In Brugg, the building produced well over 31 percent of its energy needs independently. In Sion, where the average temperatures are much lower, this figure was just under 21 percent, and over 37 percent in the much warmer Lugano. In each instance, the system saved the environment around 20 percent of CO2 emissions compared with a conventional gas heating system.
The technology pays off financially as well
One obvious question is whether the technology also makes economic sense. “When the study was published before war broke out in Ukraine, prices for electricity and natural gas were still so low that the system would not have been cost effective,” Moioli says. “But since then, the natural gas price alone has almost doubled, to around 12-15 cents per kilowatt hour. This now means our system should already be profitable, assuming a service life of roughly 20 years.”
Future price trends will certainly have a major influence on whether, and to what extent, the technology will become established. Power-to-gas competes, for example, with heat pumps, which use electricity to draw heating energy from the ambient heat of a house or from groundwater heat. These, too, have become increasingly efficient in recent years and are now replacing gas or oil heating in many homes. "They are great as an environmentally friendly solution,” says Moioli. “But they require electricity to be purchased in winter – precisely when there is very little solar power and so the price is high. Power-to-gas technology gets round this problem.”
But does the system also pay off for smaller residential buildings, such as single-family homes? “Well, it would certainly work, but would not be as efficient,” says the PSI researcher. “In a single-family home, it’s not possible to use all the waste heat from this system. The energy can be distributed more efficiently when several households are involved.”
Moioli’s study was supported by the Swiss Federal Office for Energy through the project ‘Enabling Decentralized renewable GEneration in the Swiss cities, midlands, and the Alps’.