Storage (Batteries)
- Kshiti Mishra (Deactivated)
- Gert-Jan Swinkels
Electricity generated from renewable sources such as solar PV power or wind-generated power is intermittent and fluctuates significantly over the day and across seasons, or based on local weather patterns. To consistently meet the greenhouse electricity demands while reducing the dependence on fossil fuels, electricity storage is required during surplus hours in the form of batteries, to be used during periods of deficit supply and high demand.
Batteries store electricity in the form of chemical energy. Batteries have one or more electro-chemical cells - each cell contains a positive and negative electrode, and an electrolyte in liquid/paste/solid state sealed together. The battery is connected to an external source of electricity or to an electric load. The electrolye enables the exchange of ions between the two electrodes, while simultaneously causing electrons to flow through an external circuit. Since the reactions are reversible, batteries can be ‘recharged’ by supplying an external voltage across the electrodes. Typical types of batteries, based on the electrolyte material are: lead-acid, nickel-cadmium, lithium-ion, sodium-sulphur, and sodium-nickel-chloride batteries. Battery energy storage densities for some commonly used batteries are: 30 - 50 Wh/kg for lead-acid batteries, 50 - 75 Wh/kg for NiCd batteries, and 150 - 250 Wh/kg for lithium ion batteries. Their suitable storage durations ae of the order of days.
The choice of battery type will depend on factors such as the electricity demand of the greenhouse (which will decide the total storage capacity required for the battery system), energy storage density of the battery, cycle efficiency (electricity input : electricity output), lifetime, cost, and safety considerations. Large-scale utility of battery storage has been rare up until fairly recently because of low energy densities, small power capacity, high maintenance costs, short cycle life, and limited discharge capability.
Other ways to store electricity are : pumped hydroelectric storage (where electricity is used to pump water from a lower to higher reservoir - when required, it is generated by releasing water from the higher reservoir through a hydroelectric turbine), and upcoming technologies such as compressed air storage (where electricity is used to compress air and store it underground - when required, it is generated by turbines run using the stored compressed air), or hydrogen-based storage in the form of fuel cells (which use electrical energy input to split water into hydrogen and oxygen - the stored hydrogen can be electrolysed to produce electricity and water when required), which have higher storage potential, but also high costs and larger scale of required infrastructure.
Contribution to energy balance and resource use of greenhouses:
By storing electrical power generated from renewable resources when the generation is in excess compared to greenhouse demand, batteries reduce the intermittent availability of renewable electricity to supply to components such as heat pumps, mechanical dehumidifiers, and artificial illumination. Batteries can also serve as backup power sources during grid outages, ensuring continuous operation and protecting sensitive crops.
Batteries can enable load shifting, where electricity is stored during off-peak hours when electricity rates are low and used during peak hours when rates are higher, potentially leading to cost savings.
Possible steps towards sustainable, energy-efficient greenhouses
Development of batteries with higher storage densities can meet the greenhouse electricity demand with much greater ease of installation. The electrolyte for most batteries can be toxic for the environment. Hence the ecological impact from uncontrolled disposal of batteries must always be considered.