Why Hydrogen + Lead-acid = Synergy
Why do we still have batteries at PSH? How do the hydrogen storage and battery work together? Why lead-acid?
The energy storage system at the Phi Suea House is sized based on the estimation of the energy use and load profile at the site. We have made a very detailed estimate of all appliances and machines at the site, considering all the habits and requirements of the future inhabitants as well as seasonal fluctuations in the demand, and we sized the fuel cells to cover the average demand at night during the high demand season (dry and hot climate require more cooling and water pumping).
The site is not connected to the public grid and the system has to be able to provide all the power required at any time. It does not make economic sense to include in the system a fuel cell capable of covering all eventual loads, and the fuel cells have an initial start-up period of a few seconds; we therefore decided to include a small battery storage to cover these start up periods as well as high peaks exceeding our fuel cell capacity during the night.
Graph 1
Lead-acid batteries have been the go-to solution for energy storage needs since way back in the eighteen-hundreds and have their deserved place in the market. Largely because of their use in automobiles, they actually have the largest market share of electric storage products worldwide.
Graph market share
They are popular because of their high power-to-weight ratio and their low cost. However, there are several issues commonly associated with lead-acid batteries, namely low energy-to-weight ratio and short lifetime. The lifetime of lead-acid batteries is very strongly affected by the depth of discharge per cycle (DOD, how much the of the battery capacity is discharged and recharged each cycle) and the operating temperatures. With increasing DOD and/or temperatures, the lifetime of the batteries will be shortened almost exponentially.
Graph DOD vs. cycle
My idea is to use the lead-acid batteries in combination with the hydrogen system to maximise the advantages of each.
By only using the batteries to supply short peak loads during the night, we are making the best use of lead-acid batteries’ strengths. They do a great job at instantaneously supplying high power – and do not run for extended periods of time, thus minimizing their DOD and keeping down the operating temperature. This way we can use a relatively small battery storage, keep the costs at a minimum and extend the expected battery life to over ten years, making the investment last that much longer.