With all the talk of batteries as we move towards a 100% renewable grid, have you wondered whatever happened to thermal storage? Thermal storage (ice, chilled water, hot water) originally achieved some prominence as a way to shift building load from peak to off-peak times when electricity was cheaper. However, it never really achieved broad adoption. In recent years many aging thermal storage systems have even been decommissioned as old chilled water plants have been upgraded to more efficient versions. Peak/off-peak electricity rates, while often still in place are not enough of an incentive to use thermal storage systems as originally designed.
As renewable energy increasingly penetrates the electricity grid, the carbon intensity of electricity used by buildings changes significantly over the course of each day. Similarly, wholesale electricity pricing ranges massively each day, despite most buildings not being directly exposed to this. Electric batteries are increasing in popularity and reducing in price as a response to this. Buildings themselves are thermal batteries and can shift their loads to some extent by utilising their own thermal mass. There is an even larger opportunity to use dedicated thermal storage systems to increase the amount of flexible load.
Below is a simple example schematic of an ice-storage HVAC system. While newer systems may have high system coefficients of performance (COP), a system that is 30 years old (as many are) may operate at a COP of only around 2. That is, for every unit of electricity entering the system, 2 units of cooling are ultimately used in the building. A modern high efficiency chilled water plant providing chilled water directly to air handling units may operate with COP in the realm of 4 or 5 plus.
Source: Oró et al. 2012, https://doi.org/10.1016/j.apenergy.2012.03.058
This means that to justify use of the ice plant (from a carbon emissions perspective), ice should only be made at times when the grid is less than half as carbon-intensive as when the ice is used.
The below plot shows the grid in Victoria, Australia during a typical Summer week in early 2023. It can be seen that carbon intensity does not fluctuate enough for it to be worth using the ice system.
So ice storage is not worth it right? Well, in 2023, on most weeks in Victoria, that may well be correct. However, if we extrapolate renewable energy generation to a point where it is around 80% penetration (in line with government targets), we see a drastically different grid in 2030 – see below. This shows that almost every day there is massive variation in carbon intensity of the grid to the extent that ice storage becomes attractive again.
If we look at how many days per year will likely see ice storage being beneficial based on the above logic, we get the below chart. Clearly, it’s something building owners and operators should be considering!
Remember – this is comparing a 30-year-old ice storage plant with a modern high efficiency chilled water plant. If the ice plant is upgraded to be made more efficient as well, the opportunity to utilise it becomes even greater! There are other storage mediums that could be used as well – including phase change materials that operate at the same temperature ranges as an existing chilled water plant.
The above is a carbon-only analysis. The cost piece of this is fascinating as well, with wholesale electricity prices fluctuating wildly almost every day. Buildings with these systems that many regard as relics of a bygone era are rapidly realising that while the rest of the world is grappling to install battery storage, they are already sitting on massive batteries of their own ready to play their part in reducing emissions and saving costs.
So why aren’t people doing this already? As outlined above, the carbon intensity of the grid does not yet often make it worthwhile and, even if it did, current methodologies for measuring carbon emissions would not recognise the benefits since they are based on annual average carbon intensities. From a cost perspective, most buildings are on standard retail tariffs which do not reward buildings for using more electricity when the price is low. Another barrier is that building energy rating schemes do not recognise time-of-use flexibility as part of their frameworks. If a building utilises ice storage, while the result may be lower carbon emissions and lower costs, it may adversely impact their energy rating due to system losses.
All of these things are changing however, and in the coming decade, thermal storage will likely emerge once again as a key piece of HVAC infrastructure in many large commercial buildings.
NOTE: Grid projections used in this article are basic projections assuming linearly increasing amounts of wind, solar and batteries in the grid between now and 2030.