By David Anthony and Ken Brown
Wind turbines capture the energy contained in wind. The turbine rotates a shaft which powers an electric generator. The electricity that flows from the generator can go to the wind farm’s grid connection to be consumed immediately or go to storage. We have previously discussed the advantages of storage. Let’s look at storage using hydrogen.
Water electrolysis produces hydrogen. As the electricity flows through the water in an electrolysis unit, oxygen and hydrogen are evolved as gases at separate electrodes. In a 100% efficient unit, it takes about 39 kilowatt hours (kWh) of electricity to create 1 kilogram (kg) of hydrogen. In the real world, electrolysis units are about 80% efficient at best. With an 80% efficient unit, it takes about 50 kWh of electricity to create 1 kg of hydrogen. The hydrogen is piped to a hydrogen storage unit. To avoid the high cost of compressing hydrogen or of cooling and liquefying hydrogen, a good alternative is to store the gas in a metal hydride slurry. Safe Hydrogen uses magnesium as the metal and mineral oil as the liquefying agent. With the use of small particles and a suitable dispersant, the particles will stay in suspension almost indefinitely. Using a hydriding reactor, hydrogen is absorbed by the Magnesium Slurry with suitable pressure and temperature that ensures rapid reaction. The Magnesium Hydride Slurry that is created in this reactor then can be stored in large quantities at ambient conditions. The hydriding reaction to create the magnesium hydride slurry creates heat. This heat is about 30% of the heating value of the hydrogen gas. About 10 percentage points of this heat, or one-third of the heat, can be used to perform useful work such as generating more electricity. The rest of the heat can be used for space heating or to produce hot water. Thus the hydriding step in the process can be from 110-130% efficient.
There are a number of options for the stored slurry. One, the hydrogen can be recovered on site and the hydrogen can be used to power a gas turbine-generator. The wind farm owner has the option of selling into the real time and day ahead electric market at a time and price of his choosing. Since wind blows more at night than during the day on average, and since consumers use more electricity during the day than at night, the wholesale price at night is often $0.02 per kWh or less. It was reported in Business Week in September 2009 that year to date in the Texas Grid, the wholesale price of electricity was zero or below for 11% of the time. During those times, the generation facilities on line were paying to put power on the grid. The Electric Reliability Council of Texas (ERCOT) controls the wholesale price of electricity in the real time and day ahead markets to balance generation and load. Why would generators pay to put power on the grid? Large base-load coal and nuclear plants do not want to vary their loads. Cycling the plants leads to premature wear and high costs. Wind farms get a $0.022 production federal tax credit. Until the price passes down through a negative $0.022, wind farms still receive revenue if the turbines generate power.
Another option is to use the hydrogen slurry to “firm” the wind power. Wind does not blow consistently from hour to hour, day to day, week to week, or season to season. The ISO that supervises the grid cannot count on the full power of the wind farm’s output. Typically, only 15% of a wind farm’s output can be counted on as reliable capacity—likely to be available in any given time period. This means that for a 500MW wind farm, only 75MW is counted as generating capacity by the ISO. Often, to “firm” the wind farms output, a natural gas fired plant needs to be constructed—partially negating the carbon free output of the wind farm.
With storage, the picture can be different. Below is an example of a 500MW wind farm delivering 150MW dispatchable power 100% of the time by using storage and gas turbines(GT) powered by hydrogen. In this example, the ISO can count on 30% of the wind farm’s output.
The beige portion of the power generated is stored, the blue portion is delivered by the wind turbines to the grid, and the red portion comes from gas turbines powered by hydrogen. The horizontal axis represents a probability of power going to the grid from the wind or gas turbine. About 45% of the time in the year, 100% of the 150MW will come from wind with the excess going to storage. About 40% of the time, power comes from both the wind and gas turbines. About 15% of the time, all of the power comes from the gas turbine. In any give hour or day, power may flow in any of these ways.
- Dispatchable power can demand a higher price.
- The grid connection can have smaller capacity—it no longer has be sized for maximum wind farm output.
- Firming natural gas fired plants do not need to be built.
- The gas turbines can provide the regulation that natural gas fired turbines now provide.
- The wind turbines can spin 100% of the time the wind blows (excluding the time when the weather is too violent to operate).
Ken Brown is CEO of Safe Hydrogen, LLC, a developer of safe, transportable hydrogen.