1. Environmental Physics
Chapter 6: Solar Energy: Characteristics and Heating
Copyright © 2012 by DBS

2. Introduction Solar derived
Renewable energy provides around 8 % of the world’s energy
Wind energy is the fastest growing energy resource, followed by photovoltaics
Studies suggest renewables could rise to % share by 2050
Solar derived
RES
radiant
wind
waves
hydro
biomass
geothermal
tidal

3. Fundamental Sources of Energy
FUSION (SOLAR)
FISSION
GRAVITATIONAL(PE/KE earth-moon-sun)
Fossil fuels
Wind
Waves
Biomass
Hydro
Radiant
Nuclear energy
(man-made)
Geothermal
(natural)
Tides

4. Introduction

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 use
Figure 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 neutral
Each 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 resources
N and S Dakota, and Texas have enough wind energy potential to meet all US electricity needs
A 140 x 140 mile parcel of land in Arizona covered with solar cells could meet the entire electricity needs of the US
The problem with RES:
Seasonal and time dependent
Storage problems
Price

8. Characteristics of Incident Solar Radiation
The energy from the sun reaching the earth per day: Insolation = incident solar radiation
N. Europe 600 Btu/ft2/d kJ/m2/d 79 W/m2
Equator 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/h
600 Btu x 1 W = 79 W/m2
ft x 12 in x 2.54 cm x 1m x 24 hr x d Btu
ft 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 present
Solar heated house near Chicago, IL.

11. Characteristics of Incident Solar Radiation
Energy released from the fusion of hydrogen nuclei to produce helium nuclei
Surface ~ 6000 °C
Core 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 % IR
Only ½ of this reaches surface
Absorbed by atmospheric gases
Figure 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 out
Absorbed 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. 2.13
What Happens to Sunlight?
30% Albedo
51% ??
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 earth
Most 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 space
The reradiation back to earth is called the atmospheric greenhouse effect
Earth 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/h
1kWh/m2 / day = Btu/ft2/day

18. Characteristics of Incident Solar Radiation
Insolation at earth’s surface varies between 0 and 1050 W/m2
Depends on latitude, season, time of day, cloudiness
Figure 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 altitude
Altitude is a function of latitude
Further north you are the lower in the sky the sun will be
As fall moves into winter the sunrise and sunset points of the sun’s motion across the sky move gradually southward
Figure 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 components
Insolation is usually measured on a horizontal surface
Figure 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

26. End
Review

27. History of Solar Heating
Anasazi Indianas c BC
Archimedes ‘death-ray’ c. 212 BC
National Solar Test Facility, NM °C
Can melt quarter-inch-thick steel plate in 2 minutes.

28. History of Solar Heating
19th century - Solar steam boilers produce steam to run engines
Mouchot (French) ran a printing press using solar driven steam power
Figure 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 Egypt
Provided irrigation water from the Nile
Trough-like parabolic collector which focused the sun’s rays onto a black metal pipe
Find peak output if the total area of the collector is1207 m2
e.g. average solar insolation for June = 1200 W/m2, calculate the efficiency of the plant
1207 m2 x 1200 W/m2 = 1448 kW
Assuming: (i) all solar energy converted to thermal energy of the steam, heating it to 100 ° C
(ii) air temp. = 20 °C
Efficiency = (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 Chile
Produced more than 23,000 liters per day
Figure 6.12: Solar desalination project using a cup and plastic wrap.

31. History of Solar Heating
Solar Cooking
DeSaussure (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 Cooking
1950s - Telkes’ (American) oven. The design features a fixed cooking pot and a moveable reflector.
Heating of the pot via radiation and convection

33. Question
Suppose 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 W
850W/m2 x 20/100 x Area = 240W
A = 1.41 m2

34. Overview of Solar Heating Today
Used primarily for swimming pools and domestic hot water (DHW), also space heating
Active solar system – fluid heated by the sun is circulated by a pump or fan
Passive solar system – used no external power, fluid circulates naturally
Figure 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 pools
Three types:
Active flat-plate collectors (FPCs)
Batch water heaters
Passive (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 system
FPC on roof
Backup system
Figure 6.20: Solar domestic hot water system with heat exchanger.

39. Question
What 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/d
Heat available from FPC = insolation x area x efficiency
 46,480 Btu/d = 2060 Btu/d.ft2 x 0.40 x Area
Area = 56 ft2

40. Solar Domestic Hot Water
Batch water heaters
Black tank inside an insulated box with a glass cover
Output usually flows into conventional water heater for further heating
Thermosiphon
Water flows form the collector to the tank under natural circulation
Less dense hot water rises
Thermosiphoning
Batch water heater

41. End
Review

42. Passive Solar Space Heating Systems
Passive solar space heating – house acts as solar collector and storage facility
Figure 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 fans
Sunlight 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 hrs
Sunlight 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 insulation
Solar collection (south-facing windows)
Thermal storage
3 Types of passive systems
Direct gain
Indirect gain
Attached solar greenhouse
Smith ‘Environmental Physics’.

45. Passive Solar Space Heating Systems
Direct gain
Large south-facing windows admit solar radiation
Thermal mass exposed to direct radiation absorbs radiation
Thermal mass radiates heat back into the room at night
Figure 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 performance
Figure 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 principles
Adobe brick – sand, clay, water, sticks/straw/dung
Arg-é Bam, Iran c. 500 BC

48. Passive Solar Space Heating Systems
Indirect gain
Collects and stores solar energy in one part of the house and uses natural heat transfer to distribute this heat to the rest of the house
e.g. Trombe wall
Figure 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 greenhouse
Greenhouse on south-side of house
Acts as expanded thermal storage wall
Windows must be insulated at night
Concrete floors and water filled drums used for energy storage
Figure 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) collector
Powered by pressure differences
Air flows behind corrugated metal absorber to reduce convective heat loss
Easily retrofitted addition
Figure 6.28: Thermosiphoning air panel collector.

51. Table 6-4, p. 189

52. Active Solar Space Heating Systems
Active system
Flat plate or evacuated tube collectors (thermal storage) and mechanical means of delivering heat into the living space
Working fluid may be water or air
FPC usually roof-mounted, storage tank in the basement
Auxillary heaters (electric) may be added for days with poor insolation
May 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 system
May 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 system
e.g. air system with rock storage
Figure 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. water
Pros
Air system costs less to install
Air doesn’t freeze
Cons
Not as efficient
Larger storage facility
Costs 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 months
Collector 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 A
E.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 ft2
h~ 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 days
Require 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. Summary
Solar energy system consists of collector, storage and distribution systems
Active systems use FPC through which fluid moves to transfer collected energy, pumps or fans move the fluid between collector and storage systems
Passive systems use large south-facing windows as the collector and natural means of heat transfer, thermal mass (water, rock) within the house stores the energy
Size of collector depends on solar insolation, amount of heat needed (DHW or space heating), and collector efficiency
Collectors should be tilted at angle from the horizontal equal to latitude + 10