Energy storage solutions to enhance grid economics

By Nick Ni – Verde LLC, Massachusetts, USA

Renewable energy has grown at a dramatic rate in recent years. The US Energy Information Administration has recently reported that renewable energy sources provided 9.81% of US energy consumption and 11.82% of domestic energy production for the first half of 2013, which is about 60% higher than 10 years ago.

However, along with the growth of renewable energy, the controversy never stops. The major concern lies in the real economic value after the renewable energy is integrated into the grid. Since most renewable energy facilities – especially wind and solar – are located in less populated area and far away from the grid, it is expensive to transmit the power to end-users.

In addition, the inherent intermittent and non-dispatchable features of wind/solar energy lead to a serious power idle problem, which further increases the cost of electricity production by these renewable sources. Although the technologies development and grid expansion is expected to achieve a total system levelized cost of electricity generation of $0.08/kWh for wind and $0.14/kWh for solar by 2018, this is still more expensive than natural gas, which is $0.06/kWh.

Some critics even claim that the increasing integration of wind/solar power will create more CO₂ emissions because more coal-fired power stations are needed to maintain the stability of the grid.

Energy storage is the key

They may be right, to some extent, but that doesn’t mean that there is no sense in developing renewable energy sources, because these problems can be effectively solved after the application of energy storage solutions.

With the help of energy storage systems, the excess renewable energy can be retained when power demands are low, and used to compensate the high power demands when necessary. It is estimated that the utilization efficiency of renewable energy can be increased from 30% to 60%, which is expected to cut the cost of electricity production by half. Moreover, the regenerated power from stored energy media is very stable and continuous, so no more coal power stations are needed to maintain grid stability.

The only question is: is there any sufficient energy storage solution currently available? The answer is, yes!

Batteries for energy storage

Battery systems have been widely used for excess solar power storage. The lead-acid battery is the most cost-effective battery system so far, since the related technology is mature and lead is very cheap. Its negative influence on the environment is also minimized by the high rate of lead recycling (97% of lead is recycled for producing new batteries). But the relatively short cycle life caused by electrode sulfurization does limit its application.

The lithium-ion battery has recently attracted a lot of attention, since it has a much longer cycle life and significantly lighter weight. In addition, there is no memory effect for lithium-ion batteries, so it doesn’t need periodic discharge and saves a lot on maintenance costs. However, due to the low global reserves of lithium, the manufacturing cost of such batteries is very high.

More importantly, according to a recent study conducted by researchers at Stanford University, most of the battery solutions are not suitable for wind energy storage.

‘Due to their low energy stored on electrical energy invested ratios (ESOI_e) (5–32), conventional battery technologies reduce the EROI ratios of wind generation below curtailment EROI ratios (80),’ says project leader Charles Barnhart,^[1] which means the energy that needs to store the wind power is even higher than the energy that is actually stored.

The EROI (energy return on energy investment) ratio is equal to the amount of energy produced by a technology divided by the amount of energy required to build and maintain a storage system.

‘You wouldn’t spend $100 on a safe to store a $10 watch,’ says Barnhart. Therefore, it is necessary to develop new solutions with higher energy storage efficiency for wind power.

Hydrogen for energy storage

As the lightest element on earth, hydrogen possesses the highest energy density, and it can be produced by splitting water with electricity. If a water electrolyzer is integrated with wind turbines or solar panels, the excess wind/solar power can be utilized to generate hydrogen, and stored as an energy medium.

During grid peak hours, the stored hydrogen can then be supplied to hydrogen combustion engines or polymer electrolyte membrane (PEM) fuel cell systems to meet the demand for electricity generation.

Compared to conventional battery energy storage, the hydrogen energy storage system has two significant advantages:

• The cycle life is extremely long. Hydrogen generators are usually designed for 20 years’ service term, and PEM fuel cell systems typically target 40 000 hours of reliable operation. Recent breakthroughs in the stability of PEM fuel cells conducted by Radoslav Adzic’s group at Brookhaven National Laboratory will further extend the lifetime of such systems. According to Charles Barnhart’s research mentioned above, the most feasible way to increase the ESOI_e for a battery (the PEM fuel cell can be considered as a type of battery) is to improve the cycle life. So this system is very promising to efficiently store wind generated energy.

• The raw material needed for producing hydrogen is only pure water (common water electrolysis technology requires 10–30% of alkaline in the water to increase the conductivity of water, while PEM separation technology only needs pure water). And the only emission from a PEM fuel cell is pure water. Therefore, this system is very cost-effective and environmentally friendly.

The problem with this solution is that conventional hydrogen generators require stable current and voltage for the water electrolysis process. Since the wind/solar power is intermittent, the hydrogen generators connected to this power source can’t really work properly unless extra transformers are installed, which increase the running cost.

But the recent invention of a new type of hydrogen generator conquers this obstacle. The patent-pending Verde Renewable Hydrogen System enables the hydrogen generator to run at any current and voltage, which greatly enhances the efficiency of hydrogen production under intermittent wind/solar power. In addition, the automatic control system (based on PLC chips) is mandatory to ensure ideal cooperation between each component in this system.

In summary, the integration of suitable energy storage systems with wind or solar farms could significantly enhance the utilization efficiency of renewable energy, and bring considerable financial benefits to the grid. Therefore, it is necessary to steadily continue developing renewable energy and further increase the share of it in the grid for both economic and environmental reasons.

Verde LLC designs and deploys residential, commercial, and industrial-scale electrolysers for renewable energy storage, industrial processing, transportation fuel, natural gas plant peaking and cooling, fertiliser manufacture, and distributed generation. The company – a subsidiary of Angstrom Advanced Inc – has products in operation around the world, and an extensive network with national laboratories, commercial/industrial partners, and universities.

Reference

1. Charles J. Barnhart, Michael Dale, Adam R. Brandt, and Sally M. Benson: The energetic implications of curtailing versus storing solar- and wind-generated electricity, Energy & Environmental Science (2013), Volume 6, pp. 2804–2810. Open access: http://dx.doi.org/10.1039/C3EE41973H

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