I wanted to compare the cost effectiveness and carbon efficiency of EDM to energy storage using pumped hydro, so I did the following analysis. Pumped hydro storage is about 80% efficient per cycle. First, I was interested to know how big a reservoir do you need for a viable pumped hydro storage facility so I did the following calculation. Then I did some analysis to see what the payback for the investment would need to be.

U = mgh where

- U is energy measured in Joules
- m is the mass of water in kgs 1m cubed = 1,000 kg
- g is the earths gravitational constant of approximately 10 m/s/s
- h is the height in meters through which the water is cycled

So if we want, say, a 20MW capacity for 10 hours = 200MWh

1 Joule = 1 Watt for one second

therefore 200MWh = 200,000,000 x 3,600 Joules = 720,000,000,000 Joules

therefore 720,000,000,000 = 10 x h x m or 72,000,000,000 = h x m

So if we had a reservoir at a height of 100m

m = 720,000,000 kg of a water reservoir

1 cubic meter of water is 1000kg so we need 720,000 cubic meters of a reservoir. Incidentally we would need a water source that could supply or sink 20 cubic meters of water per second depending on whether we were pumping or generating.

This would be a reservoir 300m x 300m x 8 m deep. And for those of us familiar with acres as a unit of area this would be just over 20 acres. I estimate that to build such a reservoir from concrete would take about 50,000 cubic meters or 100,000 tons of concrete and would cost about €15,000,000 to build. Adding another €5,000,000 for pipeline, plant and switchgear would bring the project to a total of €20,000,000.

So given annual maintenance and running costs of €300,000, an annual landowner payment of €200,000, and a required ROI of say 10% to bring investors on board, the annual income required for break-even for the facility would be €2,500,000 per annum or €8,500 per day allowing for some downtime.

Assuming 50% utilisation (i.e. 100MWh generated per day) this equates to €0.085 per kWh. Also a cost allowance for the 20% cycle losses needs to be factored into the cost justification so a cost of €0.105 would be required. So in order for pumped hydro storage to be financially effective a mean daily delta of €0.105 per kWh would need to exist between peak demand and peak generation times. To achieve this cost per unit stored we have factored in very high utilisation figures. These might not be justified. In terms of carbon neutrality, pumped hydro storage is very poor because it uses large amounts of concrete and iron in its construction and it loses 20% of its energy in every cycle.

How does EDM costs stack up by comparison:

If we assume that a capital budget of €10,000 to meter and control 100kW of load. This works out at €100 per kW. Let us assume that each kW of controlled load is used for 1kWh of EDM per day. If we assume a payback period of 5 years, the capital cost is €20 per kW per annum. We therefore have 350kWh of storage for a capital cost of €20 or €0.057 per unit. The consumer (and aggregator) will also require a financial incentive to participate of say €0.05 per unit. This produces a cost of €0.107 per kWh controlled.

On initial costings pumped hydro and EDM seem similar. However EDM has three significant advantages.

- EDM produces less carbon than pumped hydro storage.
- EDM requires lower duty cycle factors to be cost effective.
- EDM reduces the cost of energy to participating consumers.

All in all, I think the case for EDM versus pumped hydro storage is very strong.