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generated. The effects of short-term fluctuations from one hour to the next are lessened by load shifting and solar heating. Meanwhile, decentralized energy management is gaining in importance as much as decentral-ized power production. To ease the burden on the grid, as much solar power as possible should be consumed on site or in the immediate vicinity of the location in which it is generated.

An intelligent management system for battery power plants is indispensable for integrating storage systems into both private and large public grids.

PHOTO: TOM BAERWALD/YOUNICOS TECHNOLOGIEzENTRUM

Surpluses persist despite on-site consumption

Photovoltaics can only generate power during the day and yields are significant-ly higher in the summer. In order for PV plants to make a considerable contribu-tion to the power supply, the further in-crease in solar power generated must be underpinned by an expansion in storage capacity. As the daily and seasonal fluc-tuations in output are each compensated for by different forms of storage, a two-pronged approach, comprising a mixture of small, decentralized short-term stor-age systems and large seasonal storstor-age systems, is required to extend storage capacities. Differentiation must also be made between storage systems that maximize benefit for plant operators (i. e.

systems that are charged when the solar power cannot be consumed immediately on site) and those which are beneficial to grid operators (i. e. systems that are only charged when there is a surplus of power in the grid).

Due to the relatively high costs of storing power, it is vital that as much as possible is consumed immediately and decentral-ly. This means that the importance of on-site consumption will continue to grow.

The on-site consumption of solar power was subsidized in Germany between January 2009 and April 2012. The strong growth of photovoltaics in Germany has now led to a situation where excess so-lar power is produced in some regions during the middle of the day. Power generation is becoming regionally con-centrated during specific periods. Given that, in most cases, the power supplied by the PV plants dramatically exceeds the demand of nearby consumers, it is impossible to avoid creating such a sur-plus simply by introducing provisions for on-site consumption. As the number of new PV installations increases year on year, the number of regions where more solar power is generated than consumed is also set to rise.

Subsidizing storage

In Germany, solar power storage sys-tems are set to be subsidized by an investment grant. It has been report-ed that in order to receive funds, the storage systems must contribute to easing pressure on the grid, especial-ly that created by solar power pro-duction. Grants of between 2,000 and 3,000 euros per storage system are anticipated and a total of 50 mil-lion euros is set to be made available for the subsidies. The exact date of the introduction is yet to be decided (as of March 2013).

PHOTO: TOM BAERWALD/BAE BATTERIEN GMBH

Lead-acid gel battery

PHOTO: TOM BAERWALD/YOUNICOS TECHNOLOGIEzENTRUM

Simulating sudden drops in load, short circuits or changes in the weather ensures that storage systems are also able to function safely in extreme conditions.

Storage media

The portion of solar power that can nei-ther be absorbed by the grid nor used directly on site needs to be temporar-ily stored. Batteries are the primary con-tenders for this task, as they have proven their worth over decades of use and can be employed in decentralized systems.

Owing to topographical restrictions in Germany, cross-regional storage in res-ervoirs (pumped storage hydroelectric power stations) is only possible to a very limited degree. Compressed air energy storage represents one alternative tech-nology that, in principle, holds great po-tential, but its efficiency is still in need of improvement.

Converting solar power into chemical en-ergy (e.g. by means of electrochemical hy-drogen generation) incurs relatively high losses, but does bring with it the advan-tage that energy can be stored for long periods of time. Solar hydrogen can be converted into either electricity or heat and can also be used as fuel, directly sub-stituting petroleum products. Converting hydrogen into methane would achieve an even higher energy density and thus tap into an even greater storage capacity.

In this case, the efficiency of converting the energy does drop somewhat, but this would still be acceptable given that free sunlight is the energy’s source.

0 0 1 2 3 4 6 7 0 0 1 2 3 4 6 7

Source: Volker Quaschning, Regenerative Energiesysteme, 7th edition, Hanser Verlag, Munich2011

I. II.

Potential use of solar power in private households

5. Consumption meter

In view of the steadily falling feed-in tariffs, it is becoming increasingly important for installa-tion owners to consume as much solar power as possible in their own homes. On-site consump-tion (I.), which can be increased by means of load shifting, takes precedence over storage (II.), as this entails relatively large losses. Solar power is only fed into the grid when the battery is fully charged and the consumption devices do not require power (III.). Purchasing relatively expensive electricity from the grid (IV.) is the least favorable option, and should only be considered if the PV installation is supplying too little power and the battery is run down.

The latest developments in technology do not allow us to foresee which storage systems will triumph in the long term.

The most likely scenario will involve a mixture of small, distributed, short-term storage systems and large, seasonal stor-age systems. Ultimately, if the intention is to make photovoltaics a mainstay of German power supply, a solution must be found to store the surpluses generated in summer for use during winter.

SOURCE: zENTRUM FüR SONNENENERGIE- UND WASSERSTOFF-FORSCHUNG BADEN-WüRTTEMBERG (zSW)

Storage duration and storage capacity 1 year Several storage technologies are needed to compensate for fluctuations in the amount of solar power generated. Small quantities of power are stored in batteries in the short term, while large quantities are stored in the form of hydrogen or methane in the long term.

Batteries

The only type of instant storage currently available is that of secondary electro-chemical cells, generally known as (re-chargeable) batteries. However, the un-avoidable phenomenon of self-discharge in batteries means that they are only suited to storing solar power for short (from a few hours to a few days) and me-dium (a few weeks) periods.

Moreover, the lifespan of a battery is lim-ited by its cycle life, not forgetting that the number of possible charge cycles falls as the depth of discharge increases. The battery therefore needs to be protected against over-discharge. In lead-acid stor-age batteries, for example, full discharge converts the lead sulfate into a crystalline form which is only partly dissolved when the battery is charged again, causing per-manent damage.

What is more, the capacity that can be extracted from an accumulator decreas-es as the discharge current becomdecreas-es more powerful.

Lead-acid storage batteries are cheapest and are therefore most frequently used.

They are filled with an electrolyte of di-lute sulfuric acid, meaning that if the fi-nal charge voltage is exceeded, gassing may occur. When this happens, oxygen forms on the positive electrode and hy-drogen on the negative. These two gases then form explosive oxyhydrogen. Gas-sing also leads to the gradual loss of wa-ter, which needs to be regularly refilled.

Overall, the cycle life of lead-acid storage batteries is relatively low.

In order to increase its lifespan, the elec-trolyte can be thickened using additives to form a gel. Lead-acid gel batteries can be assembled fully sealed, meaning that they are leak proof. In this case no gas is able to escape, but lead-acid gel batter-ies may dry out as a result of gassing. A special charge controller is therefore nec-essary to manage the final charge volt-age very precisely. Lead-acid gel batter-ies have double the lifespan of lead-acid storage batteries with liquid electrolytes.

They allow around 2,000 cycles, provided no more than 30% of the capacity is dis-charged each time. If 50% of the capacity is drawn on a regular basis, lead-acid gel batteries will need to be changed after around just 1,000 cycles.

Lithium-ion batteries achieve markedly higher cycle lives. If discharged and re-charged daily, they can reach a lifespan of 20 years, equating to 7,000 charge cy-cles. Their special features include high energy densities and low self-discharge rates. They also withstand high charging currents, and can therefore be charged very quickly. These advantages currently make them ideal storage batteries for homes and electric cars. Prices for such batteries are still high, however, and will not fall until mass production levels are achieved.

Redox flow batteries

Both types of storage battery (lead-acid and lithium-ion) share the common feature that their electrodes undergo chemical conversion during charging and discharging, and therefore slowly degen-erate. Redox flow batteries avoid this. A relatively new development, these bat-teries combine the properties of the ac-cumulator with those of the fuel cell.

The reactants are each dissolved in an electrolyte and circulate separately. These two electrolytes are pumped through a cell in which ions are exchanged. This cell is divided by a membrane that only allows ions to pass through it, thus pre-venting the reactants becoming mixed.

The electrolytes that store energy in re-dox flow batteries are kept in separate tanks. As a result, the quantity of energy and the output can be scaled indepen-dently of one another. Redox flow bat-teries are characterized by their high ef-ficiency and long life expectancy.

The capacity of the redox flow stor-age systems that are shortly due to be launched on the market lies between 3 and 13 kWh. Apartment buildings and commercial establishments require larg-er units, providing an opening for those redox flow batteries that are currently offered in 200 kWh modules. Here, addi-tional modules can be added to increase the capacity.

As both lithium-ion batteries and redox flow batteries are still at an early stage of development and are relatively expen-sive, the lead-acid battery is still the most economical way to store solar power, de-spite its short cycle life.

During charging, the graphite absorbs electrons, while the metal oxide at the battery’s other pole releases electrons into the external power circuit.

In doing so, lithium ions flow from left to right (,⁾

and settle between the layers of carbon. The entire process is reversed during discharging (%^{). While}

the separator is permeable to the lithium ions, it does not allow the negatively charged counter-ions to pass through it, thereby preventing self-discharge.

The graphite and metal oxide electrodes are often made in the form of foil. An electrolyte is placed between them, through which the lithium ions are able to flow.

Lithium-ion batteries come in many forms, which vary in terms of the materials used to make the electrodes, separator and electrolyte.

(cobalt, nickel, manganese) Carbon Electron Separator

Storage systems

Overall, storing solar power in batteries is a relatively expensive enterprise. It cur-rently costs roughly as much to store a kilowatt hour of electricity as it does to generate it from sunlight. The specific costs of storage (in euro cents/kWh) are not the sole criterion, however. If the goal is to operate a battery system as profit-ably as possible, cycle life, the output of the PV plant and household energy re-quirements must also be considered.

In order to incorporate batteries into a PV system, special storage systems are required which consolidate the stor-age battery with the necessary power electronics. These have only recently be-come available on the market. They not only differ according to battery type, but also based on how they are installed.

Some systems are incorporated into the house’s AC circuit, while others are inte-grated into the PV plant’s DC circuit.

Integrating the system into the AC circuit has the advantage that as much addi-tional capacity as desired can be added at a later date, irrespective of the PV capac-ity installed. A battery inverter is needed in addition to the PV inverter, meaning that relatively high outlay is required, but such systems come with the extra advan-tage that power from the grid can be fed into them more easily, as the battery in-verter operates bidirectionally.

Incorporation into the DC circuit also has two advantages: the system costs are lower and the storage efficiency is higher.

This method requires the installation of a PV inverter and a pair of DC/DC

con-verters. They set the voltages of the PV system and the battery at precisely the level that is best for the inverter. To sim-plify matters, they can be installed in the metal cabinet that houses the battery.

Despite the high investment costs in-volved, battery capacity should be se-lected to enable as much solar power as possible to be consumed on site. By way of example, for a 4 kW plant and annual energy consumption of 4,000 kWh, a ca-pacity of 6 to 7 kWh is recommended if the quota of on-site consumption is in-tended to reach 70%. A quota of over 30%

will be virtually impossible to achieve without using storage, unless the solar power is also used to heat water. Com-binations of photovoltaics, heat pumps for water heating and intelligent energy management, for instance, can achieve on-site solar power consumption rates of up to 50%, even without storage systems.

Of course, checks should always be made to examine whether or not solar thermal installations will represent the most eco-nomical solution for the actual needs and conditions of the site.

Maximizing on-site consumption with-out considering the consequences must, however, be avoided. Care must always be taken to ensure that consumption only increases in order to raise the quota of on-site consumption and that power is not used arbitrarily and unnecessarily, as this would be counterproductive in terms of energy efficiency. On the other hand, replacing fuel, by, for example, us-ing energy-savus-ing electric vehicles on a large scale, would be a highly welcome development.

Heat accumulators and heat pumps One very simple way of storing energy is to store heat. As buffer storage is already available in some houses in the form of hot water tanks, surplus solar power could also be converted into heat by con-ducting it through an immersion heater inserted into the storage tank. As long as the production of solar power is signifi-cantly more expensive than producing hot water, this will remain a very waste-ful use of energy. Nevertheless, falling solar energy generation costs combined with rising prices for raw materials will slowly close this gap.

A far more efficient application of sur-plus solar energy is in a heat pump. If this pump is capable of generating 3 kWh of heat from 1 kWh of electricity, 1,600 kWh should theoretically be sufficient to heat a well-insulated house with a living area of 120 m² and heating requirements of 40 kWh/m². This calculation is unrealistic, however, as supply and demand do not coincide: In winter, when the greatest de-mand is placed on the heat pump, the PV plant will furnish the least electricity.

DC and AC storage system AC COnnECTED current grid via an inverter and direct current converter.

In a DC system, the battery is connected between a direct current converter and the original inverter.

A fundamental decision when choosing between storage concepts is whether to use a DC- or AC-connected system. An intelligent measuring system is needed to record the quantities of power which are generated, stored, consumed on site or fed into the grid.

A production meter is only required for DC-connected systems where it is necessary to prove the quantity of solar energy generated, e.g. in the event of bonuses for on-site consumption or partial remuneration.

AC systems may be required to be fitted with a meter (not shown) attached to the battery system showing that it feeds solar power into the grid instead of charging its battery using grid power.

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The conditions are somewhat different if the heat pump is used to provide cooling via an air-conditioning unit. In this case, the periods of energy production and consumption do correlate if power from the photovoltaic plant supplies electric-ity for heat pump cooling during the summer months. In the USA, for example, heat pump cooling is already widespread.

Irrespective of their intended use, heat pumps are not suitable for storing energy over the long term, but merely represent a component of good energy manage-ment.

Energy management

It is considerably easier to generate solar power than to store it, as this entails rela-tively complex installation procedures and unavoidable losses. In order to com-plement the storage options, as much energy as possible must therefore be consumed on site and the conditions for marketing the power must be made as favorable as possible. This situation will then replace the current practice of unre-servedly feeding power straight into the grid. If the statutory feed-in tariffs were to be abolished, or sink so low that it be-came unviable to feed all the solar power generated into the grid, this custom would die away. Grid feed-in will then only be sensible under certain circum-stances and should only be considered if other options are not available.

Load shifting can help to increase on-site consumption rates. For example, large household devices that do not require power at a given time might only be switched on when solar power is in plen-tiful supply. Washing machines, tumble driers and freezers can thus contribute to improving the coordination between de-mand for power and supply. These energy management systems need to succeed in changing the consumption patterns of the average consumer, i.e. to drive them away from simply using household pow-er at any time at the push of a button.

They must clearly indicate the costs per

kilowatt for each device at a given mo-ment and ideally offer alternative oper-ating times within the shortest possible time frame.

Favorable sales conditions will become possible if the tariff for purchasing pow-er from the grid increases while the solar power produced on house roofs becomes cheaper at the same time. This power

Favorable sales conditions will become possible if the tariff for purchasing pow-er from the grid increases while the solar power produced on house roofs becomes cheaper at the same time. This power