Solar ponds
In applications calling for large amounts of low temperature heat lt 100 C , the conventional collectors described in Chapter 5 are often too expensive. A solar pond is an ingenious collector, which uses water as its top cover. Consequently a large 'pond', of surface area perhaps 104m2 and containing Figure 6.10 In a solar pond, convection is suppressed and the bottom layer retains the heat from the sun. 104m3 of water, can be constructed with simple earthworks at low cost. Moreover, it...
Enclosed black container black tank
Here the water is enclosed in a shallow matt-black tank or bag Figure 5.1 c . So no heat is lost by evaporation. The matt-black outer surface absorbs mcdTf dt if a static mass m of fluid is being heated mc T2 T1 if a mass m flows through the collector in unit time radiation well typically a 0.9 . Some of this absorbed heat is then passed to the water inside by conduction. This type of heater is cheap, easy to make and gives moderately hot water 20 C above ambient , but may have a short...
Water vapour and air
The absolute humidity or 'vapour concentration' x is the mass of water vapour present in 1.0 m3 of the air at specified temperature and pressure. This reaches a practical maximum at saturation, so if we try to increase x beyond saturation e.g. with steam , liquid water condenses. The saturation humidity xs depends strongly on temperature Table B.2 b . A plot of x or some related measure of humidity against T is called a psychometric chart Figure 6.6 . The ratio x Xs is called the relative...
Improved solar water heaters Sheltered black container
The container of Figure 5.1 d can be sheltered from the wind and so has convective loss reduced by encapsulating it in a covered box with a transparent lid Figure 5.1 e . Glass is often the chosen cover material, having small absorptance for the solar short wave irradiation. Clear, i.e. new, polythene sheet also has small short wave absorptance and is cheaper initially, but has to be cleaned and replaced more frequently since it degrades in the open environment. Moreover, glass has a...
Info Dtu 1
Let bu2 v, where b n 4u2 and so v -nu2 2u 2. Hence u
Problems Cjm
15.1 a A cube of'hot rock'of side h has its top surface at a depth d below the earth's surface. The rock has a density p and specific heat capacity c. The material above the cube has thermal conductivity k. If the rock is treated as an isothermal mass at temperature T above the earth's surface with no internal heat source, show that the time constant for cooling is given by r phcd k b Calculate r for a cubic mass of granite of side 10 km, density 2.7 x 103 kgm , specific heat capacity 0.82 x...
Metal plate collectors with moving fluid
We now consider systems of commercial acceptability. In the plate and tube collector Figure 5.1 f , water is confined in parallel tubes which are attached to a black metal plate. It is essential to have small thermal resistance between the plate and the tube, and across the plate between the tubes. Typically the tube diameter is 2 cm, the tube spacing 20 cm and the plate thickness 0.3 cm. The plate and tubes are sheltered from the wind in a framework with a glass top and thick side and rear...
Consider The Sun And Earth To Be Equivalent To Two Spheres In Space.from The
4.1 a Consider the Sun and Earth to be equivalent to two spheres in space. From the data given below, calculate approximately the solar constant outside the Earth's atmosphere Wm-2 . b Consider the Earth as apparent from space i.e. bounded by its atmosphere to be a black body with surface temperature T. Calculate T. How does the Earth's surface temperature T' relate to T and what variables control T' Diameter of the Sun 2RS 1.392 x 109 m Diameter of the Earth 2RE 1.278 x 107m Sun-Earth distance...
Solution guide to problems at the end of chapters
1.1 a Average irradiance absorbed flux total area of Earth 1.2 x 1.2 Consider costs over 10 000 h., with an electricity price of 0.10kWh-1. a Direct cost 10x 0.5 10000hx0.100kWx 0.10kWh-1 5 100 105 dominant cost is the electricity. b Direct cost 1 x 4.0 10000 x 5 22 0.100kW x 0.10kWh-1 4 23 27 18 of a so direct savings 70 of a with higher proportion of capital cost. c Savings with b average 70 of cost of a , i.e. savings average 74 per CFL. Savings equal cost of b when 74 10000h T 4 23 10000h...
Info Gwy
i In calculating simple payback as in Example 17.1 , effectively the assumed discount rate d 0, so the discount factor 1 d - is 1.00 for all n. ii In this case the discount factor is the same for both the alternatives, so it has been applied to the difference D to calculate the NPV. That is, the NPV of the difference between the alternatives equals the difference of the NPVs. iii For n lt 5, the NPV of the solar heater is greater than that of the electrical for n gt 5, the NPV of the solar...
Heat transfer
With direct solar, geothermal and biomass sources, most energy transfer is by heat rather than by mechanical or electrical processes. Heat transfer is a well-established, yet complex, subject. However, we do not need sophisticated detail, which is rarely required to understand and plan renewable energy thermal applications. For instance, as compared with fossil and nuclear fuel engineering plant, temperature differences are often smaller, geometric configurations less complicated and 'most...
Geothermal energy
The inner core of the earth reaches a maximum temperature of about 4000 C. Heat passes out through the solid submarine and land surface mostly by conduction - geothermal heat - and occasionally by active con-vective currents of molten magma or heated water. The average geothermal heat flow at the Earth's surface is only 0.06 Wm- , with a temperature gradient of lt 30 cCkm-1. This continuous heat current is trivial compared with other renewable supplies in the above surface environment that in...
Extraction techniques hot dry rock
Sources of 'hot dry rock' HDR are much more abundant than are hydrothermal regions temperatures of 200 cC are accessible under a significant proportion of the world's landmass. This has motivated considerable research in the USA and Europe on techniques to harness this heat for electricity power generation. One result has been the recognition that few basement rocks are completely dry, but there are many regions where utilisation of their geothermal heat requires 'enhanced geothermal systems',...
Biogas
Chynoweth, D.P., Owens, J.M. and Legrand, R. 2001 Renewable methane from anaerobic digestion of biomass, Renewable Energy, 22,1-8. Advocates anaerobic digestion as the principal pathway to use of energy crops. Lettinga, G. and Van Haandel, A. 1993 Anaerobic Digestion for Energy Production and Environmental Protection, in Johansson et al. Survey with emphasis on industrial scale. Meynell, P.J. 1976 Methane - Planning a Digester, Prism Press, Dorchester, UK. An old but still useful short and...
Bibliography Undergraduatelevel books
Hall, D.O. and Rao, K.K. 1999 Photosynthesis 6th edn Edward Arnold, London. A short and stimulating introduction with more physical bias than many others. Lawlor, D.W. 2001, 3rd edn Photosynthesis Molecular, Physiological and Environmental Processes, BIOS Scientific Publications, Oxford. Concise text for biology undergraduates. Monteith, J. and Unsworth, K. 1990, 2nd edn Principles of Environmental Physics, Edward Arnold, London. Considers the physical interaction of plant and animal life with...
Batch transport
Biomass can be transported in suitable vehicles by road, rail, river or sea. However, the small density and bulky nature of most biomass as harvested means that it is rarely economic to distribute it over long distances gt 500km . Even over medium distances 100-500km , it is unlikely to be economic to distribute such biomass for its energy value alone. The guiding principle for the economic and ecological use of biomass is to interact with a 'flow' of harvested biomass which is already...
Photosynthesis at the plant level Absorption of light
Solar radiation incident on a leaf is reflected, absorbed and transmitted. Part of the absorbed radiation lt 5 provides the energy stored in photosynthesis and produces oxygen the remainder is absorbed as sensible heat producing a temperature increase, or as latent heat for water evaporation kinetic and potential energy changes are negligible . Oxygen production is a function of the wavelength of the radiation and may be plotted as an action spectrum, as in Figure 10.5 b . Figure 10.5 shows...
Structure of plant leaves
A general structure and scale of green leaves is given in Figure 10.6. In practice there is extensive variety and complexity in all these aspects down to molecular scale, as described in texts on plant physiology. In outline 1 Photosynthesis occurs in plant material, usually green leaves and algae, which we consider here. It also occurs in some simple organisms e.g. purple bacteria without the associated O2-producing system. Chloroplast cells contain organelles COV O2 guard cell Gas and vapor...
Problems Dif
9.1 From 9.16 the fraction of power extracted from the wind is the power coefficient CP 4a 1 a 2. By differentiating with respect to a, show that the maximum value of CP is 16 27 when a 1 3. 9.2 The calculation of power coefficient CP by linear momentum theory Section 9.3.1 can proceed in terms of b u2 u0. Show that a CP 1 b2 1 b 2, b CP is a maximum at 16 27 when b 1 3, c a 1 b 2 where a u0 u1 u0, and d the drag coefficient 1 b2 . 9.3 a By considering the ratio of the areas A0 and A1 of Figure...
Alcoholic fermentation Alcohol production methods
Ethanol, C2H5OH, is produced naturally by certain micro-organisms from sugars under acidic conditions, i.e. pH 4 to 5. This alcoholic fermentation process is used worldwide to produce alcoholic drinks. The most common micro-organism, the yeast Saccharomyces cerevisiae, is poisoned by C2H5OH concentration greater than 10 , and so stronger concentrations up to 95 are produced by distilling and fractionating Figure 11.6 . When distilled, the remaining constant boiling point mixture is 95 ethanol...
Social implications
The Industrial Revolution in Europe and North America and industrial development in all countries have profoundly affected social structures and patterns of living. The influence of changing and new energy sources has been the driving function for much of this change. Thus there is a historic relationship between coal mining and the development of industrialised countries, which will continue for several hundred years. In the non-industrialised countries, relatively cheap oil supplies became...
Lift and drag forces fluid and turbine machinery
Here we introduce the forces of lift and drag which are as fundamental to turbine motion as they are to sailing yachts and airplanes, for they apply to any solid object immersed in a fluid flow. Obtaining rotary motion on a shaft from a flow of water or air is the basis of every turbine, relating, in this book, to hydro turbines Section 8.4 onwards for conventional hydropower, Sections 13.4 and 13.5 for tidal power , wind turbines Section 9.2 onwards , and wave power turbines, included in...
CxHy Oz x y zHO x y zCO x y zCH
C6H10O5 H2O 3 CO2 3 CH4 11.6 Some organic material, e.g. lignin, and all inorganic inclusions do not digest in the process. These add to the bulk of the material, form a scum and can easily clog the system. In general 95 of the mass of the material is water. The reactions are slightly exothermic, with typical heat of reaction being about 1.5MJkg 1 dry digestible material, equal to about 250 kJ per mole of C6H10O5. This is not sufficient to significantly affect the temperature of the bulk...
Photovoltaic circuit properties
With photovoltaic cells, as with all renewable energy devices, the environmental conditions provide a current source of energy. The equivalent circuit portrays the essential macroscopic characteristics for power generation Figure 7.24 . From the equivalent circuit Figure 7.24 , Figure 7.14 Equivalent circuit of a solar cell. Figure 7.14 Equivalent circuit of a solar cell.
Info Xrn
Figure 9.10 Simplified drag machine with hinged flaps on a rotating belt. Figure 9.10 Simplified drag machine with hinged flaps on a rotating belt. Values of CD range from nearly zero for a pointed object, to a maximum of about 1.5 for a concave shape as used in standard anemometers. Thus maximum power coefficient for a drag machine is CPmax 27 1.5 27 22 9.40 This may be compared with the Betz criterion for an 'ideal' turbine of Cp 16 27 59 9.17 . In Section 9.4 we show that lift force turbines...
Variation with height
Wind speed varies considerably with height above ground this is referred to as wind shear. A machine with a hub height of say 30 m above other obstacles will experience far stronger winds than a person at ground level. Figure 9.17 shows the form of wind speed variation with height z in the near-to-ground boundary layer up to about 100 m. At z 0 the air speed is always zero. Within the height of local obstructions wind speed increases erratically, and violent directional fluctuations can occur...
Scientific principles of renewable energy
The definitions of renewable green and finite brown energy supplies Section 1.3.1 indicate the fundamental differences between the two forms of supply. As a consequence the efficient use of renewable energy requires the correct application of certain principles. It is essential that a sufficient renewable current is already present in the local environment. It is not good practice to try to create this energy current especially for a particular system. Renewable energy was once ridiculed by...
Anaerobic digestion for biogas Introduction
Decaying biomass and animal wastes are broken down naturally to elementary nutrients and soil humus by decomposer organisms, fungi and bacteria. The processes are favoured by wet, warm and dark conditions. The final stages are accomplished by many different species of bacteria classified as either aerobic or anaerobic. Aerobic bacteria are favoured in the presence of oxygen with the biomass carbon being fully oxidised to CO2. This composting process releases some heat slowly and locally, but is...
T Vxd
7.0 8.0 9,0 10.0 1 1.0 12.0 13.0 14.0 15.G Figure 12.10 Scatter diagram of significant wave height Hs against zero crossing period Tz. The numbers on the graph denote the average number of occurrences of each Hs,Tz in each 1000 measurements made over one year. The most frequent occurrences are at Hs 3 m, Tz 9 s, but note that maximum likely power occurs at longer periods. these waves have equal maximum gradient or slope the maximum gradient of such waves, e.g. 1 in 20 _lines of constant wave...
Conservation of momentum
Newton's second law of motion may be defined for fluids as 'At any instant in steady flow, the resultant force acting on the moving fluid within a fixed volume of space equals the net rate of outflow of momentum from the closed surface bounding that volume.' This is known as the momentum theorem. As an example, consider a fluid passing across a turbine in a pipe. In Figure 2.3, fluid flowing at speed u1 into the left of the control surface carries momentum pu1 per unit volume in the direction...
Types and adaptations of photovoltaics
Although the flat plate Si solar cell has been the dominant commercial product, there is a great variety of alternative types and constructions. These seek to improve efficiency and or to decrease the cost of the power produced by reducing capital cost. This section is a brief summary of a complex and continually changing scene Table 7.3 . 1 Single crystal. The cells described so far assume single-crystal base material. This can be produced by most of the methods of Section 7.6.2. In...
t Dws
Figure 12.18 Wells turbine. a Sketch. b Motion of a turbine blade as seen by a fixed observer . Figure 12.19 Oscillating water column device with parabolic focussing, seen from above after Energetech lt www.energetech.com.au gt . What amplification might expected from a 50 m parabola for wavelengths of 25 m, 50 m, 250 m Discuss the merits of using a bigger parabola say 200m span . Are there any natural geometries which might have a similar effect Barber, N.F. 1969 Water Waves, Wykecham, London....
Taylor Francis
Taylor amp Francis Group LONDON AND NEW YORK First published 1986 by E amp FN Spon Ltd Second edition published 2006 by Taylor amp Francis 2 Park Square, Milton Park, Abingdon, Oxon OXI4 4RN Simultaneously published in the USA and Canada by Taylor amp Francis 270 Madison Ave, New York, NY 10016, USA This edition published in the Taylor amp Francis e-Library, 2006. To purchase your own copy of this or any of Taylor amp Francis or Routledge's collection of thousands of eBooks please go to...
ADP Nhl
Figure 10.14 The Z scheme of photosynthesis at the reaction centres eight photons per C fixed. The vertical scale indicates the excitation energy of the electron. in the system is used to identify the particular centre. This is called P680 for PS2, and P700 for PS1, since the respective molecules absorb distinctively at 680 10nm 1.82 eV and 700 10nm 1.77eV respectively. Note that both these molecules absorb at the red end of the spectrum indicating their position at the lower potential energy...
Direct combustion for heat
Biomass is burnt to provide heat for cooking, comfort heat space heat , crop drying, factory processes and raising steam for electricity production and transport. Traditional use of biomass combustion includes a cooking with firewood, with the latter supplying about 10-20 of global energy use a proportion extremely difficult to assess and b commercial and industrial use for heat and power, e.g. for sugarcane milling, tea or copra drying, oil palm processing and paper making. Efficiency and...
Info Ovb
For the reflectance between air n0 1 and plastic say n1 1.6 , pref 5.3 . Semiconductors have a refractive index represented by a complex number since they are partly conducting which is frequency dependent and averages about 3.5 in magnitude over the active spectrum. Considering the radiation frequency in electron volts, the reflectance in air varies from pref 1.1 eV 34 to pref 5eV 54 . A thin film thickness t of appropriate material placed between air and a semiconductor can largely prevent...
Info Fyq
Here Cp is an efficiency factor called 'the power coefficient'. Note that the power PT is proportional to A and to the cube of wind speed u0. Thus whereas doubling A may produce twice the power, a doubling of wind speed produces eight times the power potential. The power coefficient Cp also varies with wind speed for individual machines. Since wind speed distribution is skewed, at any one time speeds less than average are more likely than speeds greater than average. Therefore the optimum...
Photochemical electricity production
The driving function of photosynthesis is the photon-induced molecular excitations of PS2 and PS1. These involve electronic excitations within molecular structure and charge separations, and are not exactly comparable with electron-hole separation across the band gap photovoltaic devices. Nevertheless these molecular excitons have sufficient energy to drive an external electric circuit, and research and development continues for photochemical power devices. A number of related devices are...
Crop drying
The drying of crops e.g. fruit, copra, cocoa, coffee, tea , for storage and subsequent sale, is commonly accomplished by burning wood and the crop residues, or by using the waste heat from electricity generation. The material to be dried may be placed directly in the flue exhaust gases, but there is a danger of fire and contamination of food products. More commonly air is heated in a gas air heat exchanger before passing through the crop. Drying theory is discussed in Section 6.4. Combustion of...
Biofuel classification
Biomass is largely composed of organic material and water. However, significant quantities of soil, shell or other extraneous material may be present in commercial supplies. It is essential that biomass is clearly assessed as either wet or dry matter mass, and the exact moisture content should be given. If m is the total mass of the material as it is and m0 is the mass when completely dried, the dry basis moisture content is w m m0 m0 and the wet basis moisture content is w' m m0 m. The...
Qi r cos aQj
c Find also an expression for the force on the plane, and evaluate it for the case b 10cm, t 1cm, Q 10L.s-1, a 60 . Figure 2.9 For Problem 2.2. A plane jet strikes an inclined wall. Figure 2.9 For Problem 2.2. A plane jet strikes an inclined wall. 2.3 Consider a steady laminar flow between two fixed parallel plates at y 0 and y D cf. Figure 2.4 . The fluid is pushed to the right x increasing by a constant pressure gradient 'dp 'dx lt 0 . a What are the forces acting on an element of fluid of...
Info Jcv
Table B.5 Miscellaneous physical fundamental constants, to 3 significant figures
Wind speed probability distributions Weibull and Rayleigh
The analysis of Example 9.1 depended solely on field data and repetitive numerical calculation. It would be extremely useful if the important function the probability distribution of wind speed, could be given an algebraic form that accurately fitted the data. Two advantages follow 1 fewer data need be measured, and 2 analytic calculation of wind machine performance could be attempted. Using the symbols of the previous section, Therefore, by the principles of calculus For sites without long...
Bibliography Monographs
Avery, W.H. and Wu, C. 1994 Renewable Energy from the Ocean - A Guide to OTEC, Oxford University Press John Hopkins University series . A substantial and authoritative study of the science, engineering and history of OTEC. Ramesh, R., Udayakumar, K. and Anandakrishnan, M. 1997 Renewable Energy Technologies Ocean Thermal Energy and Other Sustainable Energy Options, Narosa Publishing, London and Delhi. Collection of optimistic papers from a conference on OTEC in Tamil Naidu, India. d'Arsonval,...
Biological storage
The growth of plants by photosynthesis, and the consequent storage of the solar energy, is fully discussed in Chapter 10. This energy is released in the combustion of biological and, from prehistory, of fossil fuel material. The heats of combustion of many of these materials are listed in Appendix B, Table B.6. The many chemical paths for producing biofuels are described in Chapter 11, see Figure 11.2. Some of the biofuels are liquids and gases that may be used in internal combustion engines,...
Info Lko
Source Material drawn mainly from WMO, Guide to meteorological instruments and methods of observation 1996 , esp. Chapter 7 'Measurement of radiation' Notes a Integrating the signal from instruments that measure global irradiance yields insolation, b Price excluding the ancillary instruments in most cases, ancillary instruments cost at least as much again. c In essence, the 'special analysis' compares the radiation coming in from the Sun with that reflected from Earth the difference is that...
Pyrolysis destructive distillation
Pyrolysis is a general term for all processes whereby organic material is heated or partially combusted to produce secondary fuels and chemical products. The input may be wood, biomass residues, municipal waste or indeed coal. The products are gases, condensed vapours as liquids, tars and oils, and solid residue as char charcoal and ash. Traditional charcoal making is pyrolysis with the vapours and gases not collected. Gasification is pyrolysis adapted to produce a maximum amount of secondary...