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SPACE HEATING AND THERMAL STORAGE

Space heating and cooling constitutes only a modest portion of our overall electricity use [some details / pointers]. Not so modest is the contribution these make at peak load times; that is, by air conditioners on hot days and electrical heaters on cold mornings. These peaks are a significant issue for the electricity system with or without the inclusion of renewable generators. However, the inclusion of large amounts of renewable power into the generation mix, especially Wind and Solar, leads to some particular issues. These issues are outlined in what follows.

1. While not directly related to thermal storage, it can be useful to remind ourselves that the variability of renewable power sources, especially Wind and Solar, can lead to the situation where these power sources are absent (not producing power) at precisely the time that demand is maximal, thus requiring the system overall to maintain sufficient gas and other fuelled generators to meet these demands. For example, blocking high pressure systems in the Great Australian Bight (i.e. low wind) can be associated with southern heatwaves. Conversely, solar power can be reliable on hot summer afternoons. These aspects are dealt with [elsewhere].

2. While electricity itself is problematic to store, the cool or heat that we use our air conditioners and heaters to generate can sometimes be conveniently stored for many hours and even days, thus allowing a 'smart' system to load shift forward. By way of example, a house with a suitably configured concrete slab can use this (for heating or cooling) as a source of thermal storage in advance of cold mornings and hot afternoons. With a suitably configured electricity market such 'pre-loading' can allow the householder or business to use more cheap electricity and commensurately less expensive electricity, while at the system level peak loads are reduced and times of electricity abundance better utilised.

3. While Thermal Mass can be retrofitted into houses - perhaps as simply as including an insulated 44 gallon drum of water in the air-conditioning loop - it is to be hoped that new buildings, be they houses or other structures that will be heated and cooled, will be designed from the start with intelligent thermal storage capabilities. With such forethought it can even be useful to utilise passive (or rather low energy) systems to regulate the thermal masses. For example, rooftop exposure to the sun can be used to heat slabs on winter afternoons (using a basic solar hot water loop). These comments made, the ins and outs of building design and codes are somewhat beyond the OzEA scope, at least while we have more pressing analysis to push forward.

4. While resistive heating turns 100% of the energy in the electricity into heat, it turns out that Heat Pumps can be even more efficient [this is right, yes?]. To understand this consider the workings of a fridge: in this case the "coefficient of refrigeration" can be around three, which means that 100 Watts of electricity can be used move 300 Watts of heat out of the fridge, resulting in 400 Watts of low grade waste heat being expelled out the back of the fridge. A reverse-cycle air conditioner uses exactly this process to efficiently use electricity for space heating, in effect turning our example fridge inside-out. By 'trying' to make the outside colder, the abundance of 'low grade' waste heat ends up on the inside, which is exactly the desired outcome. The use of heat pumps over resistive heating is generally to be preferred for space heating from an electricity efficiency point of view, although the overall comparison will depend on the actual temperatures and heat pumps involved.

5. The times when we utilise air conditioners / heat pumps are precisely those times when they must work hardest against the outside temperature. A heat pump will produce more heat for a given amount of electricity in the afternoon while it is still 15 fifteen degrees than it will when night falls and the temperature drops to 4 degrees. The same holds in reverse for air-conditioning, with more efficient use of electricity for cooling at night time when the outside temperature is 25 degrees compared to 40 degrees in the blazing heat of the days. In both cases it is necessary to have suitable thermal mass within the house, or at least within the heating / cooling cycle, in order to obtain the benefit of these efficiencies.

Overall, intelligent inclusion and use of thermal storage into both buildings and electrical heating & cooling devices can provide significant peak shaving capabilities. It can also result in less overall electricity usage. Further, in the context of an electricity system with a high penetration of renewables, the use of electricity for space heating and cooling can in principle be timed to some extend to match with times when the renewable sources are producing power.

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fc - Sept 2010