Sustainability Journal

March 2013 Issue (files are in PDF format)

Storing electricity

A concentric ring heat store can concurrently handle two jobs, supplying the thermal needs of the buildings that it serves and also providing storage of electricity. The latter function can be controlled by the operator of the electrical grid that serves the area. It handles power supply problems associated with diurnal and seasonal load variations, with fluctuations from intermittent power sources like wind turbines and power interruptions from a power station or grid link. The two functions can be controlled independently (with caveats). Such dual-function stores also reduce the capital and operating costs for both the grid supplier and the 'City Block' system operator.

The systems can collect either summer heat or winter cold, or both. The details shown here are for heat collection but the cold collection operates the same way. The Grid Controller operates the heat pump on the left to add exergy to the store or else withdraws exergy from the core to recover it. The comparatively hot supply from the core reduces the building's heat pump duty cycle, thus returning the exergy in the form of electricity.

The rate of electricity recovery is proportional to the difference between the ON time of the buildings' heat pumps when the heat is extracted from the outer zone (the normal state) vs. the ON time when extracting it from the much hotter inner zone. The recovery is efficient because such an approach does not require an actual conversion from heat to power. Most of the recovery will occur during the winter (for a heat store) and at that time the outer zone will be very cold because it has been used as the source of building heat. The temperature difference between the two zones will be normally be greater in the winter than in the summer. That makes it possible to balance the total extracted electricity with the electricity that was used to power the upgrader heat pump. This temperature difference can be enhanced by running the upgrader heat pump in the winter because at that time the injected heat will be trapped by the temperature gradient in the core.

The upshot is that the grid operator can 'store' electricity by running the upgrader heat pump during the night in the summer (or during the day in the winter for a cold store) or else whenever a need arises to prevent a deep dip in the grid power load. The electricity can be recovered at any time in the winter or summer by temporarily closing the links from the respective core to the buildings' heat pumps. During the spring or fall those heat pumps may not be operating so the recovery process is not available but at those times the much more likely need is to handle excess supply problems, and that function is fully available. The grid operator can control the amount of power that needs to be made available by collecting capacity from whatever number of stores may be needed and the operator also has full control over the geographic location of the collection and recovery.

This ability to shift the power loads adds to the existing capacity of seasonal storage systems to reduce the winter and summer power loads. It provides distributed energy supply and storage so it reduces the capacity requirements for the grid, which no longer needs to perform a complicated (and expensive) dance to maintain a supply/demand balance for power coming from distant power stations. In the process it offers a scalable means of completely eliminating the reliance on fossil fuels for both building heating and for peaking power.

On the economics side the choice is between the current practice of increasing the generating capacity to meet the peak power demands, which costs about $5,000 per kW, or of using storage to meet those peak loads, which costs only a few hundred dollars per kW. Moreover, the electricity store can accumulate electricity at times when the wholesale cost is nearly zero and then releases that electricity when the wholesale price is high so although the store does not actually produce any net electricity it still delivers a very large economic return. The daily fluctuations occur throughout the year so for 365 days a year the store can be reused (with caveats), enabling a relatively small storage capacity to provide huge financial returns. Other benefits like the reduced grid costs and losses and the GHG emission reductions are pure gravy. Note, however, that these benefits can only be realized if the organizations that control the power grid and power generation choose to incorporate storage.