Sunday, April 03, 2011

Cryogenic Utility-Scale Power Storage for Load Leveling 50% Efficient

Charge/Store: The system operates by using electrical energy to drive an air liquefier (effectively the charging system) and storing the resultant liquid air in an insulated tank (effectively the energy store) at atmospheric pressure.
Power recovery: When the stored energy is required, the liquid air is released from the storage tank, pumped in its liquid form to high pressure, vapourised and heated to ambient temperature (using either ambient heat or waste heat); the resultant high pressure gaseous air is used to drive an expansion turbine which in turn powers a generator (collectively the power recovery system). The exhaust is cold air.
Cold recapture: As the cryogen is evaporated and returned to ambient temperature in the power recovery unit, we capture the cold exergy and store it. It is then used back into the liquefaction process. Harnessing and ‘recylcing’ the cold broadly halves the energy cost of liquefaction, increasing the round-trip efficiency of the system to ~ 50%. This can be increased further by adding in waste heat, including low grade heat to the power recovery process. _Highview
This approach to utility-scale power storage comes from the UK, where it has been used at a Scotland power plant for 9 months. It involves cooling air to liquid temperatures (-196 C) with excess power, and storing the cryogen in an insulated tank. When power is needed, the cryogen is released into a sealed space and allowed to warm and expand -- driving a turbine to generate power.
The Highview Cryo Energy System uses liquefied air or liquid nitrogen (78% of air) which can be stored in large volumes at atmospheric pressure. Liquid nitrogen is a very common commercial product, transported daily from liquefaction plant to customer; or, for larger users, produced on site.

Liquefied air has a high expansion ratio between its liquid state (-196º Celsius) and, more common, gaseous state; expanding about 700 times when regasified. As with a traditional steam engine, a cryogenic engine relies on phase-change (liquid to gas) and expansion within a confined space e.g. engine cylinder or turbine.

Since liquid air boils at -196º Celsius, ambient temperature will superheat it, creating regasification and expansion. An engine can therefore use freely available environmental heat as the heat source.

The energy density of cryogenic fluids, such as liquid nitrogen compares favourably with alternative energy storage fluids such as compressed air. Cryogenic storage also has the advantage over compressed gases in that it can be bulk stored above ground in low pressure tanks. _Highview_via_PO
Energy storage offers the opportunity to store, or ‘bank’ ‘wrong time’ energy and time-shift it to periods of peak demand or hold it in reserve. This is critical to enable the use of intermittent renewable power (e.g. wind or solar) when the customer needs it, not just when it is available. It can equally “provide shape” to must-run generation (i.e. nuclear or Energy from Waste/biomass), moving excess off-peak energy to the peak demand and pinch points. It also reduces both the capital cost and carbon footprint of electricity networks by making them more efficient; with stored reserves – not over-capacity of generation in reserve – to manage supply and demand.

Electricity supply and demand have to match on a second by second basis. Historically, the balance is achieved by significantly overbuilding generation, transmission, and distribution assets, and specifically keeping enough quick response generation in reserve. This is unlike any other commodity market, where storage/reserves are used to manage supply and demand, including unexpected imbalances.

Currently, quick response reserve generation includes (i) ‘spinning reserve’: generation plants burning coal or other hydrocarbons, but producing reduced power so as to be in a state of readiness; or (ii) gas and diesel generators: quick to start but environmentally poor.

Emission and environmental legislation, plus the simple cost of building over-capacity into the network is driving the need for a zero emission solution.

Market Potential for energy storage: ~ $600 billion over the next 10 -12 years

According to the US Dept of Energy, “big energy storage is an effective tool to improve the reliability, stability, and efficiency of the envisioned electrical grid of the future. This grid will be significantly impacted by new demands, such as plug-in electrical vehicles, increased use of renewable energies, and smart grid controls. Large scale storage technology could shave the peaks from a user or utility load profile, increase asset utilization and delay utility upgrades, decrease fossil fuel use and provide high levels of power quality, while increasing grid stability. In addition, distributed energy storage near load centers can reduce congestion on both the distribution and transmission systems.”

This technology can also be used for "waste heat to power" and for commercial cooling.This approach is almost an "off the shelf" design, requiring no new technology development. It appears to be easily scalable. Efficiencies of 50% are somewhat below that of flow cells (65%) and pumped storage hydroelectric (75%), but cryogenic storage does not require nearby mountain reservoirs (as with pumped storage hydro) or significant technology improvements (as with flow cells).

More: An optimist hopes that large scale energy storage will allow a small arid country like Israel to generate 90% of its power needs from solar. He is looking at pumped storage hydro and redox flow cell batteries, but the principle is the same when using cryogenic storage. Unfortunately for the Israeli solar champion quoted at the link, following his prescription would assure an even speedier death of the micro-state than is already in the cards due to demographics. The expense of building and maintaining such a huge photovoltaic array, compared to likely costs for small modular nuclear reactors, would be exorbitant -- without even taking into account the cost of necessary storage. The result of relying on big wind or big solar is economic devastation and energy starvation. Israel, like most advanced nations, cannot afford more of either.

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Blogger Snake Oil Baron said...

The liquifying process itself would produce a lot of waste heat right? If that can be used (assuming it is not already included) it would increase the 50% figure a fair amount, no?

Since other forms of energy have losses it is hard for me to know if this is enough of a breakthrough to be marketable and to make some alternative energy applications practical where they might not have been before. But it sounds like a very interesting idea.

10:03 AM  
Blogger al fin said...

Good point, Baron.

The company claims that the incorporation of waste heat in the regeneration stage would likely increase round-trip efficiency over 50%.

As you say, refrigeration through a phase change generates a lot of waste heat.

If the process can handle even more waste heat than its own -- and increase efficiencies proportionately -- it should find a home in the utility scale storage business.

Redox flow cell batteries are more efficient, and just as scalable, but lack some of the thermodynamic side benefits of the cryogenic approach.

11:39 AM  

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