2 Introduction Solar derived Renewable energy provides around 8 % of the world’s energyWind energy is the fastest growing energy resource, followed by photovoltaicsStudies suggest renewables could rise to % share by 2050Solar derivedRESradiantwindwaveshydrobiomassgeothermaltidal
3 Fundamental Sources of Energy FUSION (SOLAR)FISSIONGRAVITATIONAL(PE/KE earth-moon-sun)Fossil fuelsWindWavesBiomassHydroRadiantNuclear energy(man-made)Geothermal(natural)Tides
5 Wood and agricultural wastes Figure 6.1: U.S. renewable energy consumption (by source), The generation of electricity accounts for about one-half of the renewable resources used.3 % of total energy useFigure 6.1: U.S. renewable energy consumption (by source), 2003.Fig. 6-1, p. 162
6 Introduction Renewable energy resources (RES) do not produce CO2 Biomass does produce CO2 when burnt but is carbon neutralEach RES still has an environmental impact (but it is minimal compared to FF)
7 Introduction The potential of RES: Earth receives thousands of times more energy from the sun daily than is used in all other resourcesN and S Dakota, and Texas have enough wind energy potential to meet all US electricity needsA 140 x 140 mile parcel of land in Arizona covered with solar cells could meet the entire electricity needs of the USThe problem with RES:Seasonal and time dependentStorage problemsPrice
8 Characteristics of Incident Solar Radiation The energy from the sun reaching the earth per day: Insolation = incident solar radiationN. Europe 600 Btu/ft2/d kJ/m2/d 79 W/m2Equator 2000 Btu/ft2/d kJ/m2/d W/m2
9 Question Show that 600 Btu/ft2/d = 79 W/m2 A conversion factor is 1 W = 3.41 Btu/h600 Btu x 1 W = 79 W/m2ft x 12 in x 2.54 cm x 1m x 24 hr x d Btuft in cm d h
10 Characteristics of Incident Solar Radiation With current technology, the sunlight falling on a typical single house can provide from 1/3 to ½ of the heating needs anywhere in the US, even with cloud presentSolar heated house near Chicago, IL.
11 Characteristics of Incident Solar Radiation Energy released from the fusion of hydrogen nuclei to produce helium nucleiSurface ~ 6000 °CCore 40 x 106 ° C
12 Characteristics of Incident Solar Radiation Intensity of EM radiation from the sun received at the top of the earth’s atmosphere 9 % UV, 40% visible, 50 % IROnly ½ of this reaches surfaceAbsorbed by atmospheric gasesFigure 6.2: Spectrum of solar radiation reaching the earth at the top of the atmosphere and at ground level.
13 Reflected 3 %Incoming solar energy 100%Radiated from clouds + atmosphere 60%Radiated from earth 6%6%Energy in = Energy outAbsorbed by clouds + atmosphere 19 %Direct 21%Net terrestrial radiation 8%Scattered 29%Reflected 3 %Conduction/convection 33%105%113%Figure 6.3: Energy balance for the earth. The earth receives about 50% of the incident solar radiation: 21% is from direct radiation and 29% is scattered through the clouds. The energy leaving the earth’s surface comes from evaporation and conduction to the atmosphere (33%), and infrared radiation (noted here as terrestrial radiation). Most of the infrared radiation (113%) is absorbed by the atmosphere and reradiated back to the surface (the “greenhouse effect”). In order to have temperature equilibrium at the earth’s surface, the energy input must equal the energy output. For this figure, 50% (incident radiation) = 3% (reflected) + 33% (evaporation) + 14% (net terrestrial radiation: 113% + 6% − 105%).Fig. 6-3, p. 165
14 Characteristics of Incident Solar Radiation Albedo
15 What Happens to Sunlight? Fig. 2.13What Happens to Sunlight?30% Albedo51% ??19% Absorbed
16 Characteristics of Incident Solar Radiation Relatively constant temperature of the earth is a result of the energy balance between incoming solar radiation and the energy radiated from the earthMost of the IR radiation emitted from the earth is absorbed by CO2 and H2O (and other gases) in the atmosphere and then reradiated back to earth or into outer spaceThe reradiation back to earth is called the atmospheric greenhouse effectEarth temperature is maintained ~ 40 °C higher than it would be with no atmosphere (-15 °C)
17 Characteristics of Incident Solar Radiation Insolation at the top of the earth’s atmosphere solar constant = 1354 W/m2 = 429 Btu/ft2/h1kWh/m2 / day = Btu/ft2/day
18 Characteristics of Incident Solar Radiation Insolation at earth’s surface varies between 0 and 1050 W/m2Depends on latitude, season, time of day, cloudinessFigure 6.4: Motion of the earth around the sun, illustrating the seasons and the tilt of the earth’s axis. (controls latitude and season)
19 Characteristics of Incident Solar Radiation - Incoming solar radiation spread out- More atmospheric scattering- More direct incoming solar radiation- Less atmospheric scattering
20 Insolation is lowest in winter when the need for heat is highest Figure 6.5: Insolation values for a clear day on a horizontal surface located at 40°N latitude, as a function of the month and the hour of the day.Figure 6.5: Insolation values for a clear day on a horizontal surface located at 40°N latitude, as a function of the month and the hour of the day.Fig. 6-5, p. 167
21 Characteristics of Incident Solar Radiation Sun’s elevation, or angle above the horizon is called its altitudeAltitude is a function of latitudeFurther north you are the lower in the sky the sun will beAs fall moves into winter the sunrise and sunset points of the sun’s motion across the sky move gradually southwardFigure 6.6: Yearly and hourly changes in the sun’s position in the sky for 40°N. Also shown are the solar altitude θ (angle above the horizon) and the solar azimuth φ (angle from true south).
22 Characteristics of Incident Solar Radiation Insolation reaching the surface is composed of direct, diffuse and reflected componentsInsolation is usually measured on a horizontal surfaceFigure 6.7: Components of solar radiation.
23 Insolation on a vertical surface in winter is greater than on a horizontal surface Figure 6.8: Daily clear-day insolation as a function of month and collector orientation.Figure 6.8: Daily clear-day insolation as a function of month and collector orientation.Fig. 6-8, p. 169
24 Figure 6.9: Mean daily solar radiation (on an annual basis) for radiation incident on a horizontal surface, in units of Btu/ft2/d.Figure 6.9: Mean daily solar radiation (on an annual basis) for radiation incident on a horizontal surface, in units of Btu/ft2/d.Fig. 6-9, p. 169
25 To calculate space heating requirements need data on average insolation and outdoor temperatures (Climate Atlas)Table 6-3, p. 170
27 History of Solar Heating Anasazi Indianas c BCArchimedes ‘death-ray’ c. 212 BCNational Solar Test Facility, NM °CCan melt quarter-inch-thick steel plate in 2 minutes.
28 History of Solar Heating 19th century - Solar steam boilers produce steam to run enginesMouchot (French) ran a printing press using solar driven steam powerFigure 6.11: Solar steam engine, Paris, Water was heated by the sun at the focus of the concentrating dish (A). The steam produced was used to run a steam engine (B) whose mechanical output ran a printing press. The water was supplied from tank (C).
29 Early 20th Century Egyptian Solar Power Plant 1912 Shuman (American) put into operation the first large scale solar power plant in EgyptProvided irrigation water from the NileTrough-like parabolic collector which focused the sun’s rays onto a black metal pipeFind peak output if the total area of the collector is1207 m2e.g. average solar insolation for June = 1200 W/m2, calculate the efficiency of the plant1207 m2 x 1200 W/m2 = 1448 kWAssuming: (i) all solar energy converted to thermal energy of the steam, heating it to 100 ° C(ii) air temp. = 20 °CEfficiency = (TH-TC)/TH = 80/373 = 0.21 = 21%Max. useful work output = 1448 kW x 0.21 = 304 kW
30 History of Solar Heating 1872 – Wilson (Sweden) built a 4700 m2 solar still for the desalination of sea water in ChileProduced more than 23,000 liters per dayFigure 6.12: Solar desalination project using a cup and plastic wrap.
31 History of Solar Heating Solar CookingDeSaussure (Swiss) obtained temperatures high enough for cooking in a glass covered insulated ‘hot box’1860s - Mouchot’s solar pot was able to bring 3 liters of water to a boil in 1.5 hours.1870s - Adam’s solar cooking apparatus, India, Sunlight is reflected to the blackened metal container, containing the food, as shown in the insert. The metal container is enclosed in a glass jar.
32 History of Solar Heating Solar Cooking1950s - Telkes’ (American) oven. The design features a fixed cooking pot and a moveable reflector.Heating of the pot via radiation and convection
33 QuestionSuppose the solar radiation is 850 W/m2 and you can collect 20 % of the energy that falls on the reflecting surface of a solar hot dog cooker. If you need 240 W for the cooker, what is the minimum collector area required?Power required = 240 W850W/m2 x 20/100 x Area = 240WA = 1.41 m2
34 Overview of Solar Heating Today Used primarily for swimming pools and domestic hot water (DHW), also space heatingActive solar system – fluid heated by the sun is circulated by a pump or fanPassive solar system – used no external power, fluid circulates naturallyFigure 6.17: General features of a solar heating system (active or passive).
35 Solar Domestic Hot Water 5% of collectors sold today are for DHW, 95% for poolsThree types:Active flat-plate collectors (FPCs)Batch water heatersPassive (thermosiphoning systems
36 Solar Domestic Hot Water Flat-plate collector to preheat water for domestic hot water uses. The house also uses passive solar heating.Figure 6.18: Cross-section of a flat plate collector (FPC) showing heat losses and gains. Temperatures of around °F.
37 Figure 6.19: Solar collector absorber plates. Fig. 6-19b, p. 179
38 Solar Domestic Hot Water Solar DHW systemFPC on roofBackup systemFigure 6.20: Solar domestic hot water system with heat exchanger.
39 QuestionWhat size flat plate collector (FPC) is needed to supply a family’s domestic water needs in March in Denver, Colorado? Assume 80 gallons per day (1 gal = 8.3 lb), ΔT = 70 °F for the water, and that the collector-heat exchange system has an average efficiency of 40 %. The collector tilt angle is equal to the latitude (see Appendix D).Heat needed, Q:Q = mc ΔT = 80 gal x 8.3 lb/gal x 1 Btu/lb.°F x 70°F = 46,480 Btu/dHeat available from FPC = insolation x area x efficiency 46,480 Btu/d = 2060 Btu/d.ft2 x 0.40 x AreaArea = 56 ft2
40 Solar Domestic Hot Water Batch water heatersBlack tank inside an insulated box with a glass coverOutput usually flows into conventional water heater for further heatingThermosiphonWater flows form the collector to the tank under natural circulationLess dense hot water risesThermosiphoningBatch water heater
42 Passive Solar Space Heating Systems Passive solar space heating – house acts as solar collector and storage facilityFigure 6.23: The Brookhaven house: an energy conservation house at the Brookhaven National Laboratory in New York State uses a greenhouse as a major passive solar feature. Fuel consumption is about one-fourth the normal usage of a house of similar size in the same climate.
43 Passive Solar Space Heating Systems Passive solar space heating – heat flows by natural means, no mechanical devices such as pumps or fansSunlight collected through south-facing windows and the energy is stored in the thermal mass of the building (concrete, water, stone etc.)More solar energy transmitted through glass than is lost through the same windows over 24 hrsSunlight is kept out during summer using roof overhangs (sun is higher in the sky)
44 Passive Solar Space Heating Systems Essential elements of a passive solar system:Excellent insulationSolar collection (south-facing windows)Thermal storage3 Types of passive systemsDirect gainIndirect gainAttached solar greenhouseSmith ‘Environmental Physics’.
45 Passive Solar Space Heating Systems Direct gainLarge south-facing windows admit solar radiationThermal mass exposed to direct radiation absorbs radiationThermal mass radiates heat back into the room at nightFigure 6.24: Passive solar system—direct gain. South-facing windows act as solar collectors. Moveable insulation is used to cover the windows at night to reduce heat loss. A massive concrete floor acts as a storage device and prevents overheating. The overhang blocks the summer sun.
46 Passive Solar Space Heating Systems Temperature performanceFigure 6.25: The performance of a passive solar commercial building (the Conservation Center, Concord, New Hampshire) during three sunny but cold winter days. Heating was with direct gain (large double-glazed, south-facing windows, with no night insulation). Thermal storage consists of a dark slate floor over a 4-inch concrete slab and phase change materials in the walls. Even though the outside temperature ranged from 20°F down to –15°F, no auxiliary heat was used.
47 Passive Solar Space Heating Systems Adobe houses of the SW US utilize solar gain and thermal mass principlesAdobe brick – sand, clay, water, sticks/straw/dungArg-é Bam, Iran c. 500 BC
48 Passive Solar Space Heating Systems Indirect gainCollects and stores solar energy in one part of the house and uses natural heat transfer to distribute this heat to the rest of the housee.g. Trombe wallFigure 6.26: Indirect gain. The concrete wall acts as a solar collector and a heat storage medium. At night the vents are closed to prevent heat loss.
49 Passive Solar Space Heating Systems Attached greenhouseGreenhouse on south-side of houseActs as expanded thermal storage wallWindows must be insulated at nightConcrete floors and water filled drums used for energy storageFigure 6.27: Indirect gain, using an attached greenhouse. As a combination of direct and indirect gain systems, the water drums and masonry floor of the attached greenhouse provide needed heat storage.
50 Passive Solar Space Heating Systems Thermosiphoning air panel (TAP) collectorPowered by pressure differencesAir flows behind corrugated metal absorber to reduce convective heat lossEasily retrofitted additionFigure 6.28: Thermosiphoning air panel collector.
52 Active Solar Space Heating Systems Active systemFlat plate or evacuated tube collectors (thermal storage) and mechanical means of delivering heat into the living spaceWorking fluid may be water or airFPC usually roof-mounted, storage tank in the basementAuxillary heaters (electric) may be added for days with poor insolationMay be vertical mounted (~60% less insolation than roof)Figure 6.29: Basic space heating and domestic hot water system.
53 Active Solar Space Heating Systems Active systemMay be vertical mounted (~60% less insolation than roof)Figure 6.30: Active solar space heating and domestic hot water system integrated into the façade of this house in Austria, a so-called “solar combisystem.”
54 Active Solar Space Heating Systems Active systeme.g. air system with rock storageFigure 6.31: Hot-air flat plate system. Air transfers heat from the collector either directly into the rooms or into the rock storage bin (solid line). When heat is being removed from storage (dashed line), the air flow is in the opposite direction so that as much heat as possible can be picked up from storage. Water for domestic use is preheated in the storage bin.
55 Active Solar Space Heating Systems Air vs. waterProsAir system costs less to installAir doesn’t freezeConsNot as efficientLarger storage facilityCosts more over time (running costs)Difficult to retrofit (size of ducts)
56 Active Solar Space Heating Systems Sun is lower in the sky during winter monthsCollector must be positioned at a large angle (local latitude + 10)Figure 6.32: Calculating collector tilt angle from the horizontal for space heating.
57 Active Solar Space Heating Systems Optimum angle for Pittsburgh?= 50
58 Active Solar Space Heating Systems To calculate area of flat panel collector required:Require quantity of heat needed (Q), average insolation (I), efficiency of the collector (ε)Q = I x ε x AE.g. How many square feet of FPC are required to provide all thermal energy needed to heat a home for one day when the heat load is 20,000 Btu/hr? Mean daily insolation is 1800 Btu/ft2.d and efficiency is 50%20,000 Btu/hr x 24 hr/d = 480,000 Btu/d = 1800 Btu/ft2.d x 0.50 x A A = 533 ft2h~ one half of the roof!At $45 /ft2 = $24k
59 Thermal Energy Storage Solar energy heating system must be able to store energy for nighttime use and cloudy daysRequire materials with large specific heat (Q=mcΔT) (e.g. rock in the case of air heating systems)
60 Thermal Energy Storage Other media include phase-change materials, melting during day, freezing at night (releases heat)
61 SummarySolar energy system consists of collector, storage and distribution systemsActive systems use FPC through which fluid moves to transfer collected energy, pumps or fans move the fluid between collector and storage systemsPassive systems use large south-facing windows as the collector and natural means of heat transfer, thermal mass (water, rock) within the house stores the energySize of collector depends on solar insolation, amount of heat needed (DHW or space heating), and collector efficiencyCollectors should be tilted at angle from the horizontal equal to latitude + 10