In conclusion, hydraulic sources are the most easily used renewable sources and their exploitation is suggested whenever they are available. Capital investment is generally high, depending on the site and power of the plant, but it has to be considered as a long-term investment with no significant risks.
Fig. 4.4 Possible ways of converting waste into energy
Table 4.1 Typical higher heating values of waste
Description of waste
Higher heating value
kJ/kg Btu/lb
Industrial waste
Leather scrap 23,000 9,888
Cellophane 27,750 11,930
Waxed paper 27,500 11,823
Rubber 28,500 12,253
Tires 41,850 17,992
Oil, fuel oil residue 41,850 17,992
Polyethylene 46,000 19,776
Agricultural
Bark 11,000 4,729
Rice hulls 13,500 5,804
Corn cobs 19,000 8,169
Composite
Municipal 10,000–15,000 4,299–6,449
Industrial 15,000–17,500 6,449–7,524
Agricultural 7,000–14,000 3,009–6,019
energy costs, by smoothing the profile of the energy demand, by exploiting low-rate opportunities in purchasing energy, and by freeing energy recovery from user demand.
The main approaches to energy storage are hydro, mechanical, electric, and thermal.
Energy stored as primary energy in combustibles has a very high mass- energy density with typical values of roughly 45,000 kJ/kg (19,350 Btu/lb) for liquid fuels. This value is vastly higher than the storage capacity of almost any other system available in which values of 100–200 kJ/kg (45–
90 Btu/lb) seem to be the upper limit. The capital and operating costs of these systems make their extensive exploitation difficult, except for hydro energy storage, which is the most attractive and which is widely used in both utilities and industries.
Notice that both hydro and mechanical storage are mainly electric storage because the energy available is generally electric which is transformed into mechanical energy at the electric motor shaft and then stored in different forms before being transformed back into electric energy.
4.9.1 Hydro Storage
The principle of hydro storage follows the law of pumps (see Sect.10.2) and of hydraulic source exploitation by turbines (see Sect.4.7).
Generally using reversible machines, this system works in the pumping mode when an excess of energy is available (usually electric energy) and in the turbine mode when energy is required back. Water is stored in an upper reservoir and it flows back through the turbine during periods of peak demand.
Remember that energy theoretically associated with 1 m3of water and 367 m of head is 1 kWh and that the efficiency of the whole system including energy transformations in pumping mode (electric motor, pump, and flow losses) and in turbine mode (flow losses, turbine, and electric generator losses) is generally 50–60 %. It follows that the energy actually available is halved. A head of more than a hundred meters between the upper and the lower reservoir is usually required for an economic exploitation of the system.
4.9.2 Mechanical Storage
The flywheel is a typical device for the storage of mechanical energy. The quantity of stored energy depends on the shaft speed, the mass, and the radius of gyration of the flywheel (that is, the radius where the total mass is considered to be concentrated).
42 4 Utility Plants and Renewable Sources
The energy stored in a flywheel is equal to the kinetic energy, that is, Estored¼1
2 1
gcMðRΩÞ2 whereEstored¼energy stored (J),
gc¼conversion factor¼1 kgðm=s2Þ
N ,
M¼mass of the flywheel (kg),
R ¼radius of gyration (m). In the case of a disc of uniform density, with uniform thickness and outer radiusRo,R¼0.7Ro,
Ω¼revolution speed (rad/s); typical values (200–1,000 rad/s).
The energy absorbed or released from a flywheel between two rotational speeds (Ω1andΩ2) is
ΔE¼1 2 1
gcMR2 Ω21Ω22
The specific energy referred to the mass of the flywheel depends on the revolution speed, stress-to-density ratio, and geometry of the flywheel.
Typical maximum value for high-speed composite-material flywheels is 100 kJ/kg; lower values such as 20–30 kJ/kg are generally reached for low-speed flywheels with isotropic materials and constant-stress disc geometry.
4.9.3 Electric Storage
Electric energy can be stored directly by using electric batteries, among which the lead-acid type is the commonest.
The use of electric batteries for storage depends on many parameters such as charge and discharge cycle life, energy-mass and power-mass ratios, and of course energy-cost ratio.
Table4.2lists typical operating parameters of industrial batteries such as lead- acid, nickel/iron, nickel/cadmium, sodium/sulfur, and zinc/bromine.
The specific energy values range between 108 and 114 kJ/kg (30–40 Wh/kg) for lead-acid batteries and 360 kJ/kg (100 Wh/kg) for sodium/sulfur batteries; the specific power values range between 100 W/kg for lead- acid batteries and 400 W/kg for nickel/cadmium batteries; the life cycle does not exceed 400 cycles for lead-acid batteries against 1,000 cycles for nickel/cadmium batteries. The charge–discharge efficiency can reach 60–70 % depending on the operating conditions and on the recharge equipment.
These values, although higher than those of flywheel storage, are still far from the energy storage capacity of combustibles, thus limiting this kind of storage to particular applications for short periods of demand.
In addition to industrial applications, electric storage affects electric vehicle diffusion. With typical consumption per kilometer and per metric ton of 490 kJ/tkm (0.15 kWh/tkm) if measured at the utility delivering node, a wide use of these vehicles is strictly dependent on the availability of high- performance electric storage systems. With an equal division of the gross weight among payload, vehicle body and traction equipment, and batteries, the urban range is about 70–80 km; this range could be improved by using higher energy- density batteries and by reducing vehicle weight through a proper design of the body and of the traction equipment.
4.9.4 Fuel Cells
Fuel cells are electrochemical devices in which electric energy is produced by combining hydrogen and oxygen and by releasing water vapor into the atmosphere.
Basically, the main components of a fuel cell are the anode, the cathode, and the electrolyte (liquid, solid, membrane) between them. To obtain the required voltage a stack of many fuel cells in series is required.
Fuel cells can be classified according to the working temperature (see Table4.3 where the main operating parameters are reported) as (1) low-temperature cells (AFC, PEM, PEFC, PAFC) and (2) high-temperature cells (MCFC, SOFC).
Hydrogen can be produced by different techniques: natural gas steam reforming, electrolysis by using electric energy, coal gasification, and waste or biomass gasification. Table4.4reports the main operating parameters of hydrogen produc- tion and of electricity production by using hydrogen-fed fuel cells.
Table 4.2 Operating parameters of various batteries Specific energy (Wh/kg)
Specific power (W/kg)
Cycle life to 80 % (cycles)
Lead-acid 40 90 400
Nickel/iron 50 100 >1,000
Nickel/zinc 60 100 250
Zinc/bromine 50 90 200
Nickel/cadmium 50 400 >1,000
Lithium aluminum/iron sulfide
80 70 200
Sodium/sulfur 100 120 250
Iron/air 70 90 150
44 4 Utility Plants and Renewable Sources
Table4.3Operatingparametersoffuelcells LowtemperatureHightemperature AFCPEM,PEFCPAFCMCFCSOFC ElectrolytePotassiumhydroxideProtonexchange, polymericmembranePhosphoricacidLithiumcarbonate,potassium carbonateZirconiumoxide Operating temperature(C)60–12070–100160–220600–650800–1,000 (F)140–248158–212320–4281,112–1,2021,472–1,832 CatalystPlatinum,palladium, nickelPlatinumPlatinumNickelNotnecessary Materialsfor constructionPlastics,graphiteMetals,graphiteGraphite compoundsNickel,inoxsteelMetals,ceramicmaterials OxidantO2AirAirAirAir Electric efficiency(%)6040–605045–5545–60 Powerdensity (mW/cm2 )300–500300–900150–300150150–270 Powerrange (kW)5–805–250<11,000<2,000100 StartingtimeMinutesMinutes1–4h5–10h5–10h ApplicationsSpace,transportationSmallcogeneration plants,transportationCogenerationplantCogeneration,industrialplantsCogeneration,industrialplants AdvantagesHighpowerdensityHighpowerdensity, lowcorrosion, reducedstartingtime High-efficiency cogenerationHigh-efficiencycogeneration, high-temperaturerecoverable heat,internalreforming High-efficiencycogeneration,high- temperaturerecoverableheat, internalreforming,nocatalyst DisadvantagesLowresistanceto CO,highqualityof hydrogen
Lowresistanceto CO,presenceof water Lowresistance toCOShortmateriallife,CO2 recirculating,longstartingtimeHightemperature,longstartingtime Lowtemperature:AFCalkalinefuelcell,PEMprotonexchangemembranefuelcell,PEFCpolymericelectrolytemembranefuelcell,PAFCphosphoricacid fuelcell,Hightemperature:MCFCfusecarbonatefuelcell,SOFCsolidoxidefuelcell
Fuel cells, operating on non-petroleum fuels such as hydrogen, might provide an alternative energy source for electric traction and for other applications with a total efficiency, that is, combined hydrogen and electricity production efficiency, of roughly 25–40 %.
4.9.5 Heat and Cold Storage
Heat and cold storage can be classified as sensible and latent energy storage.
The first group includes systems where the storage is accomplished by increas- ing or decreasing the temperature of the material (water, organic liquid, solid); the storage energy density depends on the temperature change and specific heat of the material. The evolution of the temperature transient to reach the desired tempera- ture is roughly exponential and the time constant, that is, the time needed to reach Table 4.4 Operating parameters of hydrogen production and electricity production by fuel cells
Natural gas steam reforming
Water electrolysis
Coal gasification
Waste gasification
Biomass gasification
Hydrogen production Nm3 1 1 1 1 1
kJ 10,700 10,700 10,700 10,700 10,700
Input natural gas Sm3 0.46
kJ 16,307
Input electric energy kWh 5.8
kJ 20,880
Input coal kg 0.53
kJ 15,225
Input waste kg 2
kJ 26,000
Input biomass kg 1.4
kJ 21,000
Hydrogen production energy efficiency (a)
% 66 % 51 % 70 % 41 % 51 %
CO2local emission kg 0.9 0 1.47 1.16 0.44
Electricity production Fuel cell average efficiency (b)
% 50 % 50 % 50 % 50 % 50 %
Total efficiency (a)(b)
% 33 % 26 % 35 % 21 % 25 %
Reference values
Natural gas LHV kJ/Sm3 35,450
Coal LHV kJ/kg 29,000
Waste LHV kJ/kg 13,000
Biomass LHV kJ/kg 15,000
46 4 Utility Plants and Renewable Sources
63.2 % of the final temperature in a simplified model, depends on the product of the heat capacity of the mass involved multiplied by the thermal resistance (see Sect.
8.3) of the system.
The energy stored as sensible heat in the material at steady state, when the temperature transient has been completed, is
Estored¼cMðt2t1Þ
whereEstored¼energy stored (kJ, Btu),c¼specific heat of the material to be heated (kJ/kgK, Btu/lbF),M¼mass to be heated (kg, lb),cM¼heat capacity of the mass to be heated (kJ/K, Btu/F), andt1,t2¼temperatures of the mass to be heated or cooled (K,C,F).
If mineral oil is used for heat storage, the storage energy density associated with a temperature increase of 50 K (50C; 90F) is roughly 100 kJ/kg;
the storage energy density with water for the same temperature increase is roughly 200 kJ/kg.
If water is used for cold storage, a temperature drop of 4–5 K (7.2–9F) is usually obtained, so that the storage energy is 20 kJ/kg.
The second group includes systems where the energy is mostly stored in the form of the latent heat due to a phase change, such as melting a solid (the opposite occurs in cold storage) or vaporizing a liquid. In releasing energy, liquids solidify (or solids liquefy) and vapors condense. The storage energy density per kg, which derives mainly from the latent heat, is greater than in sensible energy storage systems. An additional advantage of this group is that it works at constant tempera- ture during the phase change.
In addition to proper transition temperatures and high latent heat, materials for storage purposes must possess other physical and chemical properties such as thermal conductivity, stability, and non-toxicity in the operating conditions. Eutec- tic salts such as NaF-FeF2and ZnC12with a density of roughly 2,000 kg/m3may reach energy densities of 400–1,500 MJ/m3or 200–750 kJ/kg (10,700–40,300 Btu/
ft3, 90–340 Btu/lb) with an overall efficiency of not more than 50 %. Capital and operating costs are quite high and do not allow a wide exploitation of this system in industry.
If ice is used for cold storage, an average storage capacity of 350 kJ/kg can be reached, of which 335 kJ/kg comes from the latent heat and 15 kJ/kg from the sensible heat. The electric energy required for producing the ice depends on the COP of the refrigerating system; if COP equals 3 (which can be considered as a typical value), the consumption is 0.032 kWh/kg of ice produced in the storage system.