Dave Elliott: Underlying the policy debate on energy is a fault line – a chasm between two basically different approaches. Not the usual one between big centralized and small decentralized energy, although that is part of it. This goes deeper. It concerns the basic, often unspoken, assumption that electricity is the key energy vector. We have the idea that electrification is modernization. It’s not just Lenin who said that, it’s everyone ever since, everywhere. It made sense. Electricity was clean, fast, controllable, and it has become increasingly valuable. However, that means it’s become increasingly expensive, in part since the main ways of producing it involved the use of increasingly scarce fossil fuels. Interestingly that, and the ever-growing environmental impacts of burning those fuels, led to drives to use it more efficiently. We have a polarity of views – essentially between backers of “pipes” and “wires”. Moving the context to the climate debate, the electric wire lobby says the energy system can best be decarbonized by sending power from wind, solar and other renewables to energy users down wires, including for heating and for charging electric vehicles (EVs). The pipe lobby says that, for heating, it makes more sense to stay with the gas grid and standard appliances but switch over to green gas. That way, you don’t have to make many changes whereas to use electricity efficiently you would have to install expensive heat pumps in every house. Green gas can also be used for vehicles, as compressed natural gas already is. So we have something of a stand-off of views. The situation is complicated by the addition of another pipe option — the supply of heat direct to users. In high-density urban environments, district heating can make more sense than individual domestic boilers, and heat networks could supply perhaps half of UK heat. What’s more, local gas-fired Combined Heat and Power (CHP) plants can supply heat much more efficiently than small domestic heat pumps. Heat pumps can have a coefficient of performance (COP) of 3 or 4, i.e. they can get three or four times more useful heat out of the input electricity than using it directly. However, CHP plants have a COP equivalent of maybe 9 or more; they use heat from burning fuel that would otherwise be wasted. The growing interest in so-called “power to gas” (P2G) hydrogen options certainly suggests that a new and large source of green gas could emerge. At present most P2G projects, for example in Germany and the UK, are dedicated to producing vehicle fuels or, to a lesser extent, gas for grid injection. But the grid-balancing role could grow and, neatly, will be made both possible and necessary by the growth of wind and solar generation. It could be a way ahead. The P2G grid-balancing approach essentially offers a way to store power until it is needed, with hydrogen or methane storage being much easier than direct electricity storage, for example in batteries. Large volumes can be stored over long times. However, there is another approach; heat can also be stored in bulk over long periods with low losses. Devotees of CHP argue that, if linked to heat stores, it too can offer a grid-balancing option, given that the ratio of power to heat output can easily be changed. When there is plenty of green power, the power output from a CHP plant can be lowered and the heat output stored, if it too is not needed. When power demand rises, the CHP plant power output can be raised, and if heat is needed, it can be supplied from the store. Integrated systems that cross the boundaries between heat and power may prove to be the way forward. But if we are to seek optimal mixes of heat and power, wires and pipes, we must move away from assuming that electricity is always the best option.
Physics World 6th March 2019 read more »