Frequently Asked Questions
Why store electricity?
Renewable generators produce electricity on an intermittent basis, requiring backup generation in networks for periods when wind or sun are not available. Renewable power cannot be generated when demand is low, so is not used. Batteries store electricity but are expensive at large scales and their efficiency declines each time they are used. TESS provides a low cost energy store can be “charged” and electricity regenerated at the very high rates needed for grid scale operators.
Is this technology suitable for individual homes?
No, the scale is better suited to entire neighbourhoods of 250 homes and above.
Is it a battery?
No, this system is a thermal energy store, not a battery.
Energy and Efficiency
What is the efficiency of a TESS?
An electrical efficiency of 31% of the energy recovery device has already been achieved with the prototype at the Tonsley workshop. The commercial TESS-IND units will have an electrical efficiency of 40%, increasing to 50% for the TESS-GRID units. The overall efficiency is greater than 80% when the heat is also utilised in food production, industry or district heating.
How is efficiency measured?
Electrical efficiency is measured as the energy charged into the system over the electrical energy produced. The overall or cogeneration efficiency is measured as the energy charged into the system over the sum of the electrical energy and useful heat produced.
How long can energy be stored?
Standard insulation allows for heat to be stored for a week. Additional insulation can extend this time, but would increase the unit cost. The TESS is designed to cycle daily, hence losses over time are not a significant driver.
Why is the heat output included in efficiency of a TESS?
A major use of electricity is producing heat for processes including, hot water, central heating and industry. The TESS can supply useful clean heat or steam for these processes without the need to convert back to electricity.
What is the efficiency of a battery?
Typical battery efficiencies are around 80%. i.e. when storing 100 units, 80 units are able to be recovered. The remaining 20% being lost as low grade heat, this can be experienced when your phone gets hot during charging. Unlike the TESS, battery efficiency decreases with each charge and discharge cycle.
What is sensible heat?
Sensible heat is the energy used to increase a materials temperature. Energy stored in rock or concrete, and graphite are examples of sensible heat storage. The efficiency of a sensible heat store decreases as it is used because the temperature decreases.
What is latent heat?
Latent heat is the energy used to change a materials phase, i.e. boiling liquid water into steam. The efficiency of a latent heat store remains constant because the temperature is constant.
What is a heat engine?
A heat engine is the general term used for any mechanical device which uses a source of heat to produce electricity, such as a gas turbine, steam turbine or Stirling engine.
About the TESS
What is thermal energy storage?
Thermal energy storage is simply the storage of energy as heat, this can be observed in its simplest form as an insulated hot water tank.
Is the TESS clean?
Yes, unlike with conventional heat engines, the TESS does not produce any exhaust gasses, as there is no combustion involved. The silicon is also non-toxic and completely recyclable.
Can the TESS store any type of heat?
No, the TESS has been designed to store electricity. The high melting point of silicon is greater than what is achievable with typical heat sources.
What is the difference between a TESS and a battery?
A battery stores electro-chemical energy while the TESS stores thermal energy i.e. heat.
How long will it last? (system lifetime)
The TESS is designed to have a service life of over 20 years. After which the majority of the components may be recycled into new units.
As it operates at a constant temperature there is little degradation of the internal components. This is contrary to batteries where the efficiency degrades with each cycle, requiring full replacement after the useful lifetime.
Does it matter where the TESS is positioned?
No, the TESS is flexible and may be positioned anywhere on the grid, off the grid, or “behind the meter” on site. This makes it particularly efficient because it can be placed where the clean heat can be used.
How long does it take to “charge”?
This is specific to each installation and the power available to charge. Generally, this time may be 1 to 8 hours.
How long does it take to “discharge”?
This is specific to each installation and the rate that energy is recovered. A typical installation would have 4 to 10 hours of storage for the rated output.
How big is a TESS unit?
The mid-scale TESS-IND unit (10MWht) is approximately the size of a standard 40 foot shipping container. The larger TESS-GRID module will be the size of a 2 story building.
Doesn’t the gas turbine burn fuel?
A gas turbine simply operates off of any hot gas such as air. In conventional gas turbines fuel is burnt to produce the hot gas.
What power output can it produce?
The modular nature of the TESS allows multiple units to be “stacked” together to provide the energy output required. Current plans are to provide from 100kW to 100 MW electrical output but there is no upper limit.
Why use silicon?
Silicon has extremely high energy density due to the latent heat properties. The high melting point also allows for high efficiency energy recovery. Silicon is also abundantly available, ensuring the technology is sustainable and affordable. It is also non-toxic and completely recyclable.
What is silicon?
Silicon is a natural chemical element (atomic number 14). It is the second most abundant element in the Earth’s crust, primarily found as a major component of common sand. Silicon like that used in the TESS is commonly used for electronic circuits. In the TESS however silicon is kept in molten form as the energy storage material.
Is silicone the same as silicon?
Silicone is an artificial substance derived from silicon and other chemicals. A rubber-like material it is typically used in sealants, adhesives, lubricants, medicine, cooking utensils, and thermal and electrical insulation.
Why does the silicon have to be melted?
To utilise the high latent heat properties which provides excellent energy storage density.
Is it safe?
Yes, the TESS is safe and has been designed to ensure safety in the unlikely event of a failure.
Is silicon toxic?
No, silicon is non-toxic.
Can the molten silicon “leak” out
No, there is no risk of the silicon leaking out. The silicon is held within specially designed containers which are stable at high temperatures. Even if the containers were to fail, thick layers of insulation prevent the silicon from escaping the unit.
How does silicon compare with molten salts?
Silicon provides much greater energy density over molten salts, as much as 2-5 times the energy storage capacity. Additionally, the high melting point increases the energy recovery efficiency.
What is a Stirling engine?
A type of piston engine which is externally heated. An internal working fluid is heated and moves between two pistons, the movement of the pistons generating electricity.
What is a steam turbine?
A steam turbine is similar to a gas turbine, rather than a hot gas steam is instead used as the working fluid.
What is a gas turbine?
A gas turbine is a type of heat engine which uses a hot pressurised gas to spin a turbine (fan blade) to produce electricity.
Doesn't the gas turbine burn fuel?
A gas turbine can operate using any hot gas including air. The TESS produces very hot air at a high temperature so it does not need a fuel such as gas.
What is pumped hydro storage?
Pumped hydro storage is the most common form of large-scale energy storage. Water is pumped to a higher reservoir or dam when power is available. When power is needed the water is allowed to flow back down, spinning a turbine in the process. It works most economically at large scale sites because of the requirements for dams which in turn have environmental issues.
What is compressed air energy storage (CAES)?
CAES is the other commercially used form of large-scale energy storage. Air is compressed and stored in large underground caverns or tanks. When power is needed the air is allowed to escape, spinning a turbine in the process. The process produces heat, but not in useful form like the TESS.