If you see me, weep
Water levels in many great rivers are dropping to critical levels and below, due to high temperatures and drought: these “hunger stones” in the Elbe (and similar ones elsewhere in central Europe) were carved in a drought four centuries ago (AD 1616) and read “If you see me, weep”. Water levels in China, Australia, parts of South America and Africa, and much of the USA, are similarly low, and aquifers depleted.
How do water, ambient heat and energy generation and storage interact? And what will be the effects on energy of the warming climate and increasing frequency of high temperatures and droughts?
As summer temperatures rise, so energy demand for air conditioning, process cooling etc. rises. In many parts of the world, electricity consumption in summer daytimes exceeds that in winter evening peaks.
The positive aspect about this increase in summer daytime energy demand is that it closely matches the power generation curve of solar. However it continues long after sunset, into the evening and sometimes overnight, therefore requiring many gigawatts of electricity storage with durations of 4-12 hours. And an increasing proportion of solar power on the grid means an increasing need for the non-energy services such as grid stability, reliability, resilience, power quality and system recovery; these are mostly best provided by plants with natural inertia, so large-scale long-duration inertial storage (of which Storelectric’s is head-and-shoulders better than the rest) will be key.
Power stations (nuclear, coal, gas, oil, biomass etc.) all use lots of cooling water to cool the turbines: the massive plumes from the cooling towers (as opposed to chimneys) is actually harmless steam. If the equipment is not cooled adequately, it overheats and fails unless it is designed (less efficiently) for hot operation.
As temperatures continue to rise, so cooling becomes harder; power stations will have to be modified for higher operating temperatures. This is already done for the hotter countries of the world, but efficiency drops. And as summer water levels in rivers and lakes fall, so increasingly power stations will be constrained in their locations, to the coasts, larger lakes and deepest rivers – though the current drying of the Rhine and Loire rivers shows that even the third of these options may be inadequate.
Hydro and Pumped Hydro
Hydro-electricity is considered with pumped hydro energy storage not only because of its commonality with pumped hydro, but also because varying the output of hydro can provide some of the energy displacement that storage would also provide. Both depend entirely on the availability of water. In such climatic conditions, not only will water be scarce but also it would evaporate fast from any reservoirs.
Large battery installations lose efficiency fast as temperatures rise. Even in temperate cooling takes up to 10% of input energy, this figure tripling through the life of the cells as they deteriorate; in hot climates the parasitic / ancillary load is much higher. Note that public quotations of battery efficiency usually exclude such loads, as also they usually exclude the 5% or so for AC-DC-AC power conversion and signal conditioning. And the hotter the cells and power electronics run, the less efficient they are and the faster they deteriorate.
Flow batteries combine the challenges of pumped hydro with those of batteries. They are large containers (up to swimming-pool size) of fluids (usually concentrated acids) in which are dissolved (usually) rare-earth metals; therefore they are subject to disruption if excessive evaporation takes place, requiring both cooling and top-up water. And cooling is also required for the power conversion and signal conditioning, which again increases with temperature.
Liquid Air Energy Storage
LAES is predicated on liquefying air and keeping it liquid. Therefore hotter climates increase the energy required to do both, with the partial compensation of a little less heat being required during the discharge part of the cycle.
Compressed Air Energy Storage
CAES proposals are of three varieties (ignoring isothermal, which is unworkable at scale), which have different considerations:
- Diabatic CAES uses turbines similarly to power stations, with their increased cooling requirement and their need for access to water;
- Adiabatic CAES (more efficient, burning no gas during discharge) with pressure compensated air storage usually has surface brine reservoirs to do that compensation: as the air is extracted, the brine is sucked into the cavern. This has the same issues as hydro and pumped hydro with water scarcity and evaporation from the reservoir.
- In the reservoir itself, assuming no water supply problem, there is a balance of benefits and challenges: the evaporation keeps the reservoir’s salinity high (which is desirable), but excessive evaporation causes salt deposition in the base of the reservoir which could clog up the system.
- If the CAES does not use salt caverns, then this is less of an issue: on the one hand there is no benefit from maintaining concentration of soluble and suspended matter, and on the other hand there would be deposition of the residual detritus and solids in suspension/solution.
- Adiabatic CAES without pressure compensation, like Storelectric’s, is very little affected by high temperatures (a little more cooling required) or water scarcity (little or none is used as a consumable).
Rising to the Challenge
As ever, the lesson is that no single challenge can be considered in isolation from all the others. Doing so jeopardises not only the energy transition but other aspects of the planet’s ecosystems. This is a macro scale of the micro problem of (for example) dealing with individual parts of the electricity system’s needs in isolation from the other needs, which is leading world-wide to exponentially increasing non-energy costs. The only realistic, deliverable and sustainable solutions deal with the challenges as a whole, in their broader context – which is the approach taken by Storelectric.