Local, decentralized energy storage permits flexibility in power generation and use. A range of technologies are available, but here Michael Lippert concentrates on battery technology as used with small-scale local generation systems, both off-grid and grid-connected, to maximize local energy use.
Historically, power grids have generally been based on large, centralized power stations that supply end users via long-established transmission and distribution networks. This traditional model has performed very well in delivering secure and reliable power. But the demand to increase the penetration of renewable energy sources is changing the nature of the grid. A growing percentage of our energy will be provided by wind farms or PV (photovoltaic) plants, often far away from population centres. Smaller embedded on-shore wind farms and community-based CHP (combined heat and power) schemes will also be more common within the distribution system.
To ensure that this new type of grid can receive power of all qualities from diverse sources, and deliver reliable consumer supply on demand, it will need to incorporate fully integrated network management. This is where the need for energy storage comes in.
ENERGY STORAGE ROLES
Energy storage will play three key roles in the new decent-ralized grid:
Dispatching energy – Energy storage makes energy available when needed, independent from the actual time of generation. This provides a better match between supply and demand as well as shifting availability from low value to high value periods, potentially enabling the generator to receive a better return on investment.
Bridging power – By balancing power flow fluctuations, energy storage can compensate for the short term intermittency of renewable energy sources. It can also help to bridge between power generation modes during the ramp-up/down periods. In addition, it can provide spinning reserve. Depending on the application, the time for which the bridging power is required may vary from seconds to minutes.
Stabilizing power – Energy storage can perform a key power quality function by providing voltage stabilization on a standby basis while also providing permanent regulation services.
ESA (Electricity Storage Association) has identified a wide variety of possible energy storage technologies for various applications, including batteries, super-capacitors, flywheels and pumped hydroelectric power schemes.
A solar-powered lighthouse with back-up power supplied by Ni-Cd batteries
In the remainder of this article we are focusing on battery-based energy storage technologies that already exist for grid applications, and will consider their use in the three main roles identified: dispatching energy, bridging power and stabilizing power.
One of the most well-known applications for battery energy storage is in off-grid PV systems that provide power for remote radio and telecom relay systems, as well as maritime signalling and DC power installations in the oil and gas industry. Batteries provide energy during periods without sunshine, e.g. during the night. They are sized to power the equipment for several days, or up to several weeks in bad weather conditions. Normally, the batteries can be fully recharged during the summer, but are often operating at a partial state of charge during the winter, when there is only sufficient daylight for a partial recharge.
A Ni-Cd battery adapted for renewable energy applications
Energy storage is also playing a key role in rural electrification in developing countries – covering both small-scale home systems providing the basic power needs for a single household, or in larger PV mini-grids serving a small community. A typical off-grid household in the developing world consumes 1 kWh of energy per day, which corresponds to a theoretical panel size of 100 W. This is expected to increase to 300 W in the future.
The system needs to ensure three days of autonomy, based on an average daily cycle of 20% DOD (depth of discharge). This calls for an energy storage system of 0.5–1 kWh per household. Typical battery requirements for off-grid and rural electrification projects include:
- capability to provide 12–48 V, supplying up to 100 kWh
- long periods of autonomous operation, ranging from several days to weeks
- daily cycles of around 10%–15% DOD
- ability to operate at partial state of charge over long periods
- high charge efficiency
- high reliability and availability
- good cycling capability
- minimal maintenance
- favourable lifecycle cost.
In addition, off-grid installations in outdoor (often remote) sites are expected to provide:
- ability to operate at high temperature with little effect on operating life
- good capacity availability at low temperature, no risk of freezing or other damage at extremely low temperatures
- good resistance to electrical and mechanical abuse
- low weight and volume for ease of transportation and installation.
The two main battery technologies currently used in off-grid PV applications, tubular lead-acid and nickel-cadmium (Ni-Cd), have been in use for well over 100 years. So there is considerable experience in failure modes and reliability issues.
Lead-acid batteries offer the lowest initial cost. However alkaline batteries, including Ni-Cd as well as the newer Ni-MH (nickel-metal hydride) technology, can offer considerable advantages, including: longer life (both calendar and cycle life), greater robustness with no risk of ‘sudden death’ failure, and lower maintenance. So although the first cost of an alkaline battery may be more expensive, a TCO (total cost of ownership) analysis will usually show it to be a more cost-effective option.
Single battery cells have relatively small voltages and capacities. This means that an energy storage battery is built up from multiple units, and it is therefore possible to construct a wide range of sizes, from small (tens of watts) to very large (MW).
Lead-acid and Ni-Cd batteries are simple to install. In general, they only require bolted connections and there is no need for special installations or infrastructure, other than ensuring that normal safety precautions are taken. They are easy to service, and service requirements are well-documented and based on many years of practical experience. Furthermore, they have a tolerant charge and discharge profile that is well suited to the vagaries of renewable energy systems.
Whatever type of battery is selected, it must be optimized and adapted for energy storage applications. It is impossible to take a standard consumer product, such as an automotive battery, and expect to achieve a reliable, long-life system.
Grid-connected PV with energy storage
PV installations which have a permanent connection to the electricity grid are categorized as ‘on-grid’ applications. This is the most popular type of PV system for homes and businesses in the developed world, comprising more than 90% of all PV installations, and by 2012 the global market is expected to reach 11 GW of PV installations per year (source EPIA).
A typical 5 kW peak panel (about 50 m²) in southern Germany delivers 4000–5000 kWh/year – sufficient to supply nearly all the annual electricity needs of an energy-conscious household. However, the current practice is to inject all of the PV energy produced into the local electricity network, to be sold to the utility. Electricity is then imported from the network to satisfy the household’s demand.
In the future, it is expected that households will become energy autonomous, producing and consuming their own electricity, with the role of energy storage being to store any excess PV energy until it is needed. Effectively, this ‘time-shifts’ PV energy produced during the day, peaking at noon, to make it available on demand. This both maximizes local consumption and enhances the efficiency of the PV system. Only surplus energy would be fed back into the grid, for which the owner of the PV system might be remunerated at a higher tariff during peak demand periods.
The indications are that future legislation in Europe will favour this type of ‘self-consumption’, especially as the clear indication of the change in energy value and availability throughout the day will encourage households to adopt a much more energy conscious attitude.
Energy storage can also increase security of supply while making individual consumers less dependent on the grid. It will help to boost the development of energy self-sufficient houses and buildings and contribute to the continuous growth of PV as part of the global energy mix.
For utilities, the main benefit of on-grid energy storage, is that it will reduce the peak load on the grid while making PV a source of predictable, dispatchable power that can be called on when needed. There is also the potential to defer costly grid upgrades.
The anticipated implementation of smart metering and real time pricing will enhance the use of both demand side management and power on demand – two major tools to help balance load versus demand in future distribution networks. With such market mechanisms in place, end users can play an active role in optimizing energy consumption, whilst maximizing the ROI (return on investment) of their PV system. Energy storage enables them to do this without any reduction in home comforts.
Operational model for on-grid energy storage
A typical residential PV system with a panel size of 5 kW produces a daily average of 14 kWh per day in Northern Europe. About 50% of the PV energy will be used as soon as it is produced, the remaining 50% can be stored until needed. Therefore an energy storage system needs to ‘shift’ between 2 and 10 kWh per day – averaging 7 kWh.
In grid-connected applications, the newest practical battery technology, lithium-ion (Li-ion), offers the potential for significant improvements in terms of performance and service life, and is also zero-maintenance. However, although Li-ion batteries are very well established in consumer applications, the more rigorous demands of PV applications means that ordinary consumer battery cells are not suitable – instead, a new generation of Li-ion battery system designed specifically for industrial applications is under development, with the first system already on field trial.
The initial indications are that Li-ion technology will offer very high efficiency (around 95%) combined with a long calendar and cycle life – 20 years at 60% DOD/day. The compact, sealed-for-life design of Li-ion batteries offers considerable advantages.
Battery energy storage in hybrid systems and micro-grids provides an effective and reliable method of ‘bridging’ between generation methods, to ensure continuity of power as one generator ramps up and the other ramps down. In cases where the final back-up generation is a diesel generator, then a battery system can help to minimize run time and fuel consumption.
Energy storage has a role to play across the spectrum of grid stabilization applications, from the few seconds required for flicker compensation, to the hours of support required for load levelling. Battery-based systems are focused on these main applications:
- compensation of voltage sags and short power cuts – seconds to minutes
- providing spinning reserve – up to 15 minutes
- uninterruptible power and black start – up to 15 minutes
- participation in the regulation market – 15–30 minutes.
A schematic for a typical domestic instalation
Battery energy storage can offer a number of advantages such as: enabling high local penetration of RES (renewable energy schemes), avoiding shutdowns, and removing the need for conventional spinning reserve.
Typical battery requirements include the capability to supply several MW, combined with a long calendar and cycle life.
- Storage technology makes energy available where and when it is needed.
- Decentralized energy storage offers the greatest added value to various stakeholders.
- Various energy storage solutions exist to cover a multitude of needs in terms of power (kW to MW) and discharge time (seconds to days).
- Battery energy storage can help to enable the penetration of renewables.
- Battery energy storage can ensure stabilization of decentralized grids.
Michael Lippert is marketing manager with Saft’s Industrial Battery Group, Paris, France.
Signalling of abandoned off-shore platforms in the North Sea
Sabik, the navigation aid specialist, has installed Ni-MH battery modules in innovative, self-contained, photovoltaic-powered navigation lights developed for marking abandoned offshore structures in the Frigg North Sea oilfield.
The 12 V, 500 Ah battery system, (pictured above installed on an abandoned North Sea structure) is required to store sufficient energy from the light’s PV modules in the spring and autumn months to ensure reliable winter operation. The Ni-MH batteries, which are sealed for life, were installed to meet Sabik’s requirement that the entire system should provide four years of maintenance-free operation.
Deployment of grid-connected energy storage in Guadeloupe
A rooftop PV installation in Guadeloupe
A current, two year project taking place on the Caribbean island of Guadeloupe is testing the viability of using Li-ion batteries in conjunction with PV systems. Fifteen PV systems are being deployed over 10 sites, each consisting of an array of solar panels and a 48 V, 10 kWh Saft Li-ion battery system that provides buffer storage for the grid-connected PV units.
During peak periods, the PV systems provide a controlled injection of 4 kWh daily to the grid upon utility demand – one hour in the morning and three hours in the afternoon, simulating the substitution of fuel-powered generators.
The first results during the summer of 2008 showed that the average daily cycle for the batteries is 45% DOD. This corresponds to about 50% of the generated PV energy stored at a battery efficiency of 97%. The expected payback time on the investment is between six and 10 years, depending on the prevailing cost of peak power.
In 2005, a four-month expedition led by the explorer Jean-Louis Etienne to Clipperton (an uninhabited Pacific Ocean atoll) relied on Saft Li-ion batteries. Power for the 25-man expedition to study flora and fauna was provided by two 3 kW solar panels and two 1 kW wind turbines, stored in two identical Li-ion batteries, rated at 6 kWh. The batteries had to supply power for refrigeration equipment, communications systems, and the desalination plant, and supplied an average 4.5 kWh each day, with a depth of discharge close to 76%, in temperatures of up to +40°C.
Etienne calculated that if his expedition had used lead-acid batteries, the weight would have been three tonnes. Instead with Li-ion batteries it was 250 kg.
The Alaska BESS
The Alaska BESS is the world’s most powerful battery
In 2003, the Golden Valley Electrical Association (GVEA) in Fairbanks, Alaska, energized a US$30 million BESS (battery energy storage system) designed to stabilize the local grid and reduce its vulnerability to outages.
The BESS, designed by ABB and Saft, comprises nearly 14,000 Ni-Cd cells to provide a nominal voltage of 5000 V and a storage capacity of 3680 Ah. The complete battery weighs around 1300 tonnes and occupies a space of 940 m2. During its commissioning tests, the BESS delivered a 46 MW discharge for five minutes to earn official recognition by Guinness World Records as ‘the world’s most powerful battery.’ The BESS is configured to operate in several distinct modes, each of them aimed at stabilizing the GVEA system if power supply problems occur. According to GVEA, the BESS typically prevents around 300,000 customer disconnections every year.