Thomas Febry

Energy Storage

Abstract

This document aims at giving a description of the different means to store energy, and description of Ireland's and European countries different means to store energy.

Energy can be stored before being produced, in the form of fossil or biomass fuels. However, energy production is never equal to energy demand, and an energy exceed must be stored. Most countries use hydroelectric stations with pumped-storage (water is pumped to the upper reservoir during off-peak hours) to store energy exceeds. However, other technologies are currently being implemented (such as compressed air energy storage) or tested (hydrogen storage).

Overview

Nowadays, there are many ways to produce electrical energy. We can burn fossile fuels, such as petrol or coal, or harvest renewable energies,such as wind or solar radiation. There is currently a debate on how we can replace little by little nuclear and fossile fuels by renewable and green energies to produce electricity. However; energy demands are always fluctuating : it generally increases during winter (for electrical heating) and lowers during summer, Moreover, we need more energy during day rather than during night.

Fig 1. Mean power generation and power load during a day. [1]

Energy production cannot be adapted to a everchanging demand : energy exceed must be stored otherwise it is wasted. As fuels are increasingly scarce, energy cannot be wasted. Moreover, wind power plants and solar panels productions cannot be controlled, and nuclear plants energy production cannot be fully controlled (there is always a minimum energy production, to maintain the chemical reaction). We need to store when they produce exceed energy.

First, I will explain how energy is transported, then I will develop the different means of storing energy, and I will compare Ireland's means of storage to other European countries' ones.

How is energy transported?

Every plant produces electricity, so this is the departure point to see how to store energy.

Electricity is the best way to transport energy as it presents numerous assets :

Fast and convenient energy transport

Electrical energy is almost instantly delivered, and it is transported into overhead power or underground power lines. Other forms of energies can be used but they present some drawbacks. For instance, in a car, motor's motion is easily transferred to wheels,, but they are just distant of several meters; this cannot be implemented in thousands of kilometers distances : investment would be too big, and frictional resistance would be too high (all the more as they would need a lot of maintenance). Electrical energy transmission just only need some power lines.

Power transmission

The plant produces power (P) which is transported by electricity; at a certain voltage (u) and at a certain current (i). the relation between these characteristics is : P= u*i. Therefoer, as power is fixed by the plant's production; if the current is fixed, voltage is fixed too. Power loss during transport by power lines is mainly due to Joule's effect (Pj) ,described by the relation Pj= R*i²; where R is the line's resistance (depending on its length; its section and its nature), Those lines are usually made of aliminium, cheaper and lighter than copper, but a bit less efficient. Lines' length is fixed, and sections are variable (from 12mm² to 750mm²a bigger section implies a lower resistance) . Usual choice is to have the current as low as possible to lower Joule's effect losses. Therefore, voltage is very high: in Ireland it goes up to 400kV in transmission power lines. Voltage is gradually decreased with transformers to finally arrive at the consumer's house at 230V[2].The efficiency of that system is around 92%[3].

Fig 2.Power transmission. [4]

easily converted into other forms of energy

Electrical energy can be easily converted into other forms of energy: into motion with a motor, into heat with a radiator (a resistance), into radiation with a lamp, into wind with a fan and into hydraulic energy with a pump.

Therefore, electrical energy is the key of energy distribution. It is almost instantly delivered, it can transmit much energy and is easily adapted to ant need. However, electrical energy is either consumed or wasted : it cannot be stored in this form.

How is energy stored?

Using fuels

Fossil fuels

The first natural idea about energy storage is to store fuels. Indeed, instead of storing energy, why not storing the fuel that produces energy? Fuel storing is the most common way to store energy, and to trade it. It is much easier to trade concrete objects, such as oil, gas or coal, rather than abstract objects such as energy. Hydrocarbon fuels are produced from organic decomposition . Buried dead organisms are very slowly decomposed, over millions of years, to finally form what we call fossil fuels, which are natural gas, coal and oil.

Fig 2.Fossil fuels creation. [5]

As we cannot produce fossil fuels, they are a limited resource and sooner or later fossil fuels must be replaced. Therefore, storing energy becomes increasingly important, as fossil energy is partially being replaced by other means of energy production that are not fully controlled (such as solar panels).

Biomass fuels

Biomass refers to everything that is organic, and that can burn or decompose. For instance, wood is a biomass fuel. Vegetables use the sun light to grow, it is the photosynthesis; when they are burnt, this absorbed energy is partially released. Therefore, biomass fuels are a renewable energy[6].

Biomass fuels are mainly turned into two fuels : ethanol and bio diesel. Ethanol is obtained by fermenting (just like in beer brewing), bio diesel is obtained from vegetable oils (beetroot, colza), algae, animal fats. Rubbish can also be used as a fuel.

Fig 2. Biomass concepts. [7]

The idea is to create fuels from renewable sources, so energy can be easily stored, either in biomass form (wood, crops...) or in fuel form (ethanol, bio diesel). However, the main drawbacks of biomass fuels is that we use food potential crops to produce energy, and that it takes time to produce crops (and then to produce fuels).

Comparing fuels

To compare each fuel, we introduce the gross calorific value, which is the energy contained in 1 kg of fuel, or in 1 m³ for a gas.

Fig 3. Fuels comparison. [8]

Using mechanical kinetic energy

Inertia moment

To make an object rotate at a given rotation speed, the energy required depends on the inertia moment. The inertia moment depends on the mass of the object, and the distance of the mass from the rotational axis. Indeed a mass far from the rotational axis will produce a greater inertia moment than the same mass near the rotational axis.

Fig 4 . Moment of inertia. [9]

The inertia moment is J = 1/2*MR² in this case.

Flywheels

Energy can be stored into motion energy using what we call flywheels. A flywheel is a wheel with a great inertia moment. Indeed energy stored depends on the rotational speed, and on the moment inertia. Increasing rotational speed or moment inertia will increase the energy stored. The energy stored is given by the formula E =1/2*Jw² where J is the inertia moment, and w is the rotational speed. To maximize the energy stored, the materials must have a high density.

The energy stored goes from 3,3kWh to 133kWh. The efficiency of a flywheel is based on the apparel that produces the rotational movement to load the flywheel, and the apparel that produces electrical energy when the flywheel unloads. I will assume that the movement generator is a synchronous motor (efficiency ~ 95%) and the electrical generator is an alternator (basically it's asynchronous motor working as a generator, so efficiency ~ 95%) . The flywheel stores rotational energy so everything is done to lower as much as possible the friction ; I assume that the frictional loss of a flywheel can be neglected compared to motor's efficiency . Therefore, the efficiency of a flywheel is around (95%)² ~ 90%. The main asset of flywheel is their fast response time, they load in several minutes, and unloads in several minutes too. Flywheels are ergonomic, as they are small, but they have a huge weight according to their length.

Fig 5. NASA flywheel module. [10]

Using mechanical potential energy

Hydraulic pumped-storage plants

Hydraulic plants use the potential energy of water,stored in a reservoir,to make a turbine turn and produce electricity. During off-peak hours, Energy exceed can be used to pump water to the lake.

Fig 6. Hydroelectric plant with pumping return. [11]

This storing method's efficiency is based on the efficiency of the turbine and the alternator that produces electricity, and on the efficiency of the pump, that stores water in the reservoir. To calculate the global efficiency, I assume that a Pelton turbine is used (efficiency near 100% [12]), the average efficiency of an hydraulic pump is 80% and an alternator efficiency is 95%. Therefore the global efficiency is 76%. Energy stored is directly linked to the energy exceed, and is 76% of it.

The time response of this storage method is the time that water takes to go up to the reservoir and to go down to the turbines, which depends on the section of the pipe leading to the reservoir, and on the characteristics of the pump. For instance,Turlough hill pumped-storage station can generate full power in 13 seconds[13].

However, sites allowing pumped-storage are limited, and creating the reservoir can endanger the local environment.

Compressed Air Energy Storage

The principle is to compress air in order to store energy during off-peak hours, and to expand the compressed air to recover the stored energy during peak hours. The air compression is adiabatic, the compressed air reaches the high pressure cavern and the compression heats the air. When energy is needed, the air must be heated (we can recover and use the heat generated by the compression), expands and go through a turbine-generator to produce energy. Air's heat is then recovered for future air expansion.

Fig 7. Hydroelectric plant with pumping return. [14]

The overall efficiency is around 70% mainly due to heat loss in the cavern. Using adiabatic compression and the biggest cavern possibles enhance the efficiency, as a small pressure change will store more energy, for little changes in temperature and heat losses[15]. The main drawback of this station is its siting: it needs a cavern with a high pressure and thermically isolated.

It can also be coupled with a plant. For instance, the first compressed air energy storage station was built at Huntorf, in Germany, and is coupled with a natural gas plant. The compressed air does not go directly to the turbines, it is first heated up by the natural gas. It works just like a classic natural gas plant, but the required natural gas is divided by 3[16].

Using Thermal energy

Thermal energy storage uses off-peak electrical energy to produce ice, chilled water or hot water. These elements are stored in thermically insulated tanks, to be used later for air conditioning or heating. They work as heat or cold tanks. Therefore, they are located to near populated areas such as towns or campuses. This can be coupled with a steam turbine generator station to produce electricity.

The energy stored is directly linked to the material characteristics, to the dimensions of the tank, and to the temperature difference between tank's interior and exterior's environment: Q = mc_p(T_int-T_ext) with m the mass of the material (linked to its density and the tank dimensions), c_p the material specific heat. In the case of a phase change ( a vaporization), the energy stored is : Q = m_liqc_pliq(T_int-T_vap)+L(liq->gas)+m_gasc_pgas(T_vap-T_ext) where liq refers to the liquid characteristics, gas to the gas characteristics, T_vap is the temperature of vaporization of the material and L(liq->gas) the latent heat for the material"s vaporisation.

Fig 8 . Solar heating coupled with molten salts thermic energy storage. [17]

I will take the example of the molten salts thermic energy storage. the salt is kept liquid in the cold tank at 288°C, is heated, and is stored in the hot tank at 566°C (there is no phase change). When electricity is required, hot salt goes to the steam generator, and steam produces electricity thanks to a turbine generator. The heat tanks efficiency is nearly 99% [17]. However the steam turbine generator efficiency is around 55-60%. For a tank of a height of 9,1m and a diameter of 24,4m, full of hot molten salts, it would store 400MWh[17].

Using Chemical energy

Hydrogen

Dihydrogen can be obtained from several manners:

- Steam reforming : At 700-1000°C, steam reacts with fossil fuels to form dihydrogen and carbon dioxide. For instance, steam can react with methan: CH₄+2H₂O → CO₂+4H₂. This reaction is exothermic, it yields energy.
- Water electrolysis : Energy exceed is used to produce dihydrogen with water electrolysis. The reaction is : 2H₂O → 2H₂ + O₂. The efficiency of the reaction is around 65-70%[18].

Fig 9 . Water electrolysis [19]

Biomass gasification; The biomass fuel (wood, straw or grass) is heated progressively (from 200°C to 1000°C) in an environment very poor in dioxygen, to avoid combustion; this is called a pyrolysis. The fuel decomposes in products that react in a partial oxidation: C_nH_m + ⁿ/₂ O₂ → n CO + ^m/₂ H₂first,then nCO + nH₂O → nCO₂ + nH₂.

Hydrogen can be stored as pressurized gas or in liquid form.

The gas is pressurized to a pressure up to 690 bar. It then contains 30kg/m³ of dihydrogen.[20] The main drawback is that compressed hydrogen tanks are very expensive, and have a huge volume and a huge weight.Storing hydrogen in liquid form improves the density of dihydrogen stored: it contains 70kg/m³. However, it needs huge quantities of energy to liquefy dihydrogen. Indeed, in order to be fully liquid, dihydrogen must be stored under -252°C with a pressure above 12,8 bar.

Other means to store dihydrogen are under development including chemical storage (storing dihydrogen using metal hybrids or carbohydrates) and cryocompression. The cryocompression is simply storing dihydrogen in liquid form in a tank tolerating high pressure. Then, when the dihydrogen receives heat from the environment, its pressure increases.

Fig 10 . A liquid dihydrogen tank [20]

Hydrogen combustion reaction is: 2 H₂(g) + O₂(g) → 2 H₂O(l) + 572 kJ . The dihydrogen molecular mass is 2 g/mol. Therefore, compressed dihydrogen contains 15000 mol/m³, and liquid hydrogen contains 35000 mol/m³ , Finally,compressed hydrogen stores 4290 kJ/m³ ( = 1,19 kWh/m³ ) and liquid hydrogen stores 10010kJ/m³ ( = 2,78 kWh/m³)

Fig 11 . Comparison of hydrogen storage methods.[21]

The volumetric capacity is the mass of dihydrogen stored per liter.

The gravimetric capacity is the percentage of the stored dihydrogen mass over the total mass.

Liquid hydrogen with cryo compression is actually the best solution to store dihydrogen, as it contains more dihydrogen for a given weight than the other methods.

Batteries

Battery principle

A battery relies on a oxydoreduction reaction. During this reaction, electrons are freed. Electrons and ions movement generates electicity.

Fig 12 . Copper/zinc battery cell in generator mode.[22]

We will focus on the second type of batteries, which are the rechargeable batteries.Theses batteries have two modes.

The generator mode, where the battery frees stored energy. For the copper/zinc example, copper is reducted (it absorbs electrons) where as the zinc is oxided (it frees electrons).

The recharging mode, the battery acts as an electric receptor: the zinc receives electrons and is reducted, the copper frees electrons, and is oxided.

There is plenty of different rechargeable batteries: lead-acid batteries, NiCd batteries, NiMH batteries, Lithium-ion and lithium-polymers batteries. They can be sorted according to their energy density.

Fig 13 . Rechargeable batteries energy density comparison.[23]

We must also take in account their efficiency and their lifetime( number of cycles) .

Fig 14 . Efficiency and battery lifetime comparison.[24][25]

Lithium-Polymer and Lithium-Ion have better characteristics than other batteries, but they cannot be used for relatively high voltage and high current devices. Indeed, Lithium batteries are used for small devices such as cell phones or laptops, and NiMH, Lead-acid batteries are used for car batteries, that requires more power. Batteries are often compared to super-capacitors. Those are electronical components, that acts as a classic capacitor, but with a higher capacity. Super-capacitors have a great lifetime, a good efficiency but a very low energy density (5 kWh/kg).

Storage comparison

Energy stored, efficiency weight and size

Fig 14 . Rechargeable energy density comparison.[26]

This chart sorts energy stocking means by their weight and size.According to this chart, Lithium ion batteries are the most efficient, because of their small size and their light weight. Flywheels are very heavy, because the energy stored is directly linked to their weight.[27]

Energy storage and costs

Fig 15 . Rechargeable energy cost comparison.[28]

This chart sorts every energy storage method by their cost. Two criteria are used: the power regularity (Capital cost per unit power) and the energy stored ( Capital cost per energy output). For instance, flywheels can store little quantity of energy according to their fabrication cost, and they have difficulties to send a regular power (the output power is not regular, it is high at the beginning of the discharge, and low at the end of discharge). According to this chart, pumped storage and CAES (compressed air energy storage) are the more adapted for huge energy storage. Indeed, their output power can be fully controlled, and they can store huge quantities of energy, according to the investment. For tiny energy storage, batteries are more adapted.

Energy storage in Ireland

Ireland's energy is mainly produced by thermal power plants and hydroelectric stations [29]. The most noticeable example is Turlough Hill hydroelectric station.Ireland plans to focus on wind energy, because of its appropriate location for oceanic winds.

Turlough Hill

Turlough Hill is a hydroelectric power plant, implementing pumped storage. It is located south of Dublin, on the Liffey River. Its construction started in 1968, and ended in 1974. Its upper reservoir can store up to 3 200 000 m³ of water. This station has 4 generators, each producing 73MW[30]. According to the station characteristics (volume of water stored : 2 300 000m³, depth h = 681 (Turlough Hill lake height) - 431 (Lough Nahanagan height) = 250m), E =water mass*g*h = 1,6 GWh : the station can store up to 1,6 GWh .Therefore, this station can produce 272MW during roughly 6 hours. Its construction cost 22 million irish pounds, which roughly equals to 200 million euros nowadays.[31]

Fig 16. Turlough Hill.[32]

Comparison with European countries

Actually, the European leader in renewable energies and storage is Germany. They developed a Compressed Air Energy Storage system at Huntorf, in 1978. It is a CAES model coupled with natural gas. This plant still works, and produces 270MW. It is also the country that has the most hydro-electric power plants, using pumped-storage. Germany also built many solar photovoltaic panels, where as its lighting is slightly identical to Ireland's lighting. Germany also promotes the use of hydrogen, as a car fuel and to store energy. Italy is currently planning to test Compressed Air Energy Storage[33].

Worldwide, countries such as France,United-States and Brazil are increasingly interested in biomass fuels, as these produce renewable energies, and can be easily traded. However, the current main technology to store energy is hydro pumped-storage, for its high capacity, very low response time and its rather low price (regarding the quantity of energy stored).

Conclusion

Energy production is hardly equal to energy demand: there is always an energy exceed. We can see energy storage in two different ways : storing the energy exceed to avoid energy wastes ( with pumped-storage for instance), or directly storing energy production fuels (such as biomass).

Nowadays, hydroelectric plants with pumped storage are the main method to store energy production exceeds, as it enables to store high quantities of energy for a rather low price, with a good efficiency. However plenty of new energy storage systems are currently being tested, such as the Compressed Air Energy Storage (its main drawback is to find a suitable location) or the use of hydrogen, as a means to store and transport energy. Another solution could be to produce, store and trade fuels (which can be obtained with biomass). On a lower scale, energy can be stored

Ireland needs to store energy exceeds, but the main difficulty is the country's size. Indeed, a small country size implies a limited use of compressed air energy storage and hydro pumped-storage technologies, as both requires specific and huge areas. Though, a hydroelectric plant with pumped storage was implemented in Turlough hill a few decades ago, and is still efficient. Ireland could also use hydrogen to store energy.

The main challenge for Ireland will be space management, as hydroelectric stations and CAES need plenty of space and are likely to modify the local environment.