Energy storage

Energy storage

Rammed earth trombe wall uses thermal mass to store solar energy.

Solar energy has traditionally been stored as heat in thermal storage systems or chemically in batteries. Solar energy has been experimentally stored thermochemically in phase change materials and at high temperatures using molten salts. The storage of excess solar energy allows for the availability of this energy during hours of darkness or cloud cover.

Thermal mass systems use various methods and materials (adobe, earth, concrete, water) to store solar energy for short or long durations (Seasonal thermal store). Thermal mass can be used to lower peak demand, shift time-of-use to off-peak hours and reduced overall heating and cooling requirements.

Solar energy can be stored thermochemically with phase change materials (PCM). Devices of this type which store latent heat can be thought of as heat batteries. Phase change materials are classified as organic (paraffins, fatty acids) and inorganic (salts, metals,alloys).

A Paraffin wax thermal storage system consists of a solar hot water loop connected to a paraffin wax tank. During the storage cycle, hot water flows through the storage tank melting the paraffin wax. The enthalpy of fusion for paraffin wax is 210-230 kJ/kg. During the heating cycle, stored heat is extracted from the tank as the wax resolidifies. These systems heat air and water up to 64°C and can reduce conventional energy use by 50%-70%.

Eutectic salts such as Glauber's salt can also be employed in thermal storage systems. Glauber's salt are relatively inexpensive and readily available and can store 347 kJ/kg and deliver heat at 64°C. The Dover house in Dover, Massachusetts was the first to use a Glauber's salt heating system in 1948.

Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are non-flammable, non-toxic, low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems. A molten salt storage system consists of a salt loop connected to a insulated storage tank. During the heating cycle the salt mixture is heated from 290°C to 565°C. During the power cycle the salt is used to make steam for a steam-electric power plant. The Solar Two used this method of energy storage. The Solar Two could store 1.44 TJ (400,000 kWh) in its 68 m³ storage tank and had an annual storage efficiency of about 99%.

Rechargeable batteries can be used to store excess electricity from a photovoltaic (PV) system. This type of storage system consists of a PV power source connected to a battery bank via a charge controller and inverter. Lead acid batteries are the most common type of battery associated with PV systems because of their relatively low upfront costs and high availability. Lead acid batteries have an energy density of 110-140 kJ/kg, a charge/discharge efficiency of 70-92% and cost $150-200 per kWh ($45-55 per MJ). Batteries used in off-grid applications should be sized for 3-5 days of capacity and should limit depth of discharge to 50% to minimize cycling and prolong battery life.

Excess electricity from PV systems can also be sent to the transmission grid. This electricity can be used to fill existing demand or temporarily stored for later use. Net metering (Grid-tied electrical system) policies give PV system owners a credit for the electricity they deliver to the grid. This credit is used to off-set electricity provided from the grid when the PV system cannot fill demand.