Comparing Energy Storage, Like with Like

One of the biggest challenges facing those who wish to invest in, or otherwise work with, energy storage is how to compare the costs and performance of different technologies. Many (e.g. batteries) carefully select their data points to flatter both their capabilities and their costs. This is a checklist of how to compare their efficiencies and costs on a like-for-like basis. To this end, they should be compared:

1. Over a 40-60-year life
   • Pumped Hydro (PHES) and Compressed Air (CAES) would include a mid-life overhaul / refurbishment
     • The PHES reservoirs and the underground portion of CAES have potential lives in excess of 100 years; the best way to accommodate this may be to discount the cost of such assets, so only a proportion of them is included in the costs
   • Batteries would include swap-outs of both cells and power electronic systems (e.g. inverters) every 8-10 years
   • Other technologies (and other aspects of these technologies) would be treated in comparable ways

1. Grid-to-grid, including:

1. • Energy transformation, both into and out of the plant
     • Substation, inverters, signal conditioning
     • An alternative figure to be given additionally, for installations in which the input power is direct current, e.g. solar, wind, interconnectors
   • Heating, cooling and ancillary loads
   • Buildings, land, design, project development, planning (these relating more to capital costs than to efficiency)

1. Efficiency should be calculated lifetime-average, and a graph provided as to how that varies during its life, e.g. deteriorating until refurbishment / cell swap-out, then re-set to a new value and resuming its deterioration afterwards
2. In different environments, as the performance of some technologies is more affected by ambient conditions than other technologies. Each climate’s cycles of temperature and humidity (and possibly other criteria) should be defined. Example climates are:

1. • Sub-Arctic, e.g. Scandinavia, much of Canada and Russia
   • Temperate, e.g. United Kingdom, Netherlands, Germany, New England
   • Continental: warm dry summers and cold winters
   • Hot, e.g. southern central USA, Middle East, central India, most of Australia
   • Monsoon season 

1. Response times to deliver 10%, 50% and 100% of output power

1. • From an “off” state
   • From spinning / zero output power
   • From delivering 10% output power

1. On one full cycle per day
2. Self-discharge, over separate periods of 1 day, 1 week, 1 month and 1 year
3. All of the above should be evaluated for different size specifications, in a 3-dimensional matrix, because most technologies are affected strongly by size:

1. • Power in: standardise at 1, 10, 50, 200 and 1000 MW
   • Power out: standardise at 1, 10, 50, 200 and 1000 MW
   • Duration, measured as hours’ output at nameplate output power, at 0.25, 0.5, 1, 2, 5, 12, 24 and 168 hours (168 hours = 2 weeks)

Multi-Tasking Technologies

In considering different storage technologies, it is important to determine the number of services that the storage can deliver concurrently, using the same resources (i.e. not sub-dividing the plant’s output or storage capacity), e.g. one of Storelectric’s plants can deliver concurrently a range of services that require many same-sized batteries:

• Balancing services and arbitrage (based on power and duration);
• Ancillary services (based on speed of response);
• Inertial services (based on inertia, and including related services, such as phase-locked loops) –
  • Distinguish between real and synthetic inertia, the former being best for preventing failures and the latter for recovering from them, as shown here;
• Reactive power and load;
• Voltage and frequency regulation;
• Black start (without having to reserve capacity);
• Other services, e.g. constraint management, curtailment avoidance.