SOLAR ENERGY I

Solar Energy is an Energy Flow  1 kW∙hr = 3.6 x 10 6 J  1 BTU = J

What do you consider to be solar energy? A.Photovoltaics B.Wind C.Hydroelectric dams D.Biofuels E.Solar collectors F.All of the above

Solar Energy is an Energy Flow  Many different forms of energy ultimately come from solar Solar heating Photovoltaic Wind Hydroelectric Biomass OTEC Other?

Should you be allowed to install solar energy units on your property? A.Yes B.No

Should new construction be required to incorporate energy? A.Yes B.No

Breakdown of Solar Energy

How Much Energy is Available?  Solar constant is 1368 W/m 2 or 2(cal/min)/cm 2.  When averaging over the whole planet, reduce by ½ because half of earth is night, reduce by another ½ because earth is curved. (This is still at top of atmosphere.)

Approximately 49% of the sunlight at the top of the atmosphere is absorbed by the surface. (We’ll usually assume 50%)

Power Density at the Surface  Average energy at the surface has been reduced by a factor of 8.  Average power over the entire planet is approximately 170 W/m 2 or 0.25(cal/min)/cm 2  The power density depends on where you are, but this is a global average.

Average Energy Density for a Full Day  170W/m 2 ×24 hr=4.08 kWhr/m 2.  Solar Insolation: Amount of solar radiation reaching the earth’s surface.  Solar insolation varies with time of day, time of year, location on earth, weather.

Solar Insolation at 40 N Latitude

Total Solar Energy in US.  If we sum the total solar insolation over the entire US we receive approximately 5.610 19 Btu/year.  We use approximately 110 17 Btu of energy per year.  Thus, we get around 525 times more energy from the sun than we use every year. If we had 100% efficient solar collectors, we could run the country and still only cover 0.16% with solar collectors!

Solar Heating  Active systems: use pumps or fans to move air or a fluid around Have moving parts Consume energy  Passive systems: have no moving parts or power use Rely on absorption of energy in mass, density differences, or natural windflow

Solar Heating: Active system  Fluid is forced through a collector by a pump or blower.

Advantages of active systems  Compact components  Flexibility of placement/storage units  Easy to control. Disadvantages: Requires power to operate it Has moving parts

Active Solar Collectors

Selecting the proper angle is important for optimal collection For a fixed panel, make sure it points south (in the Northern Hemisphere.)

What should the angle be for June? A.18.4º B.71.6º C.47.1º D.45º

Increased Solar Collection by proper tilting of panel.

Solar Heating: Passive System  Use natural convection currents and architectural features to circulate heat:  No external energy required, but very little flexibility.

Examples of Passive Solar Elements  Trombe Wall

Roof overhang and thermal mass flooring/walls

Room Arrangement

Natural Ventilation

Insulation  Reduce the flow of energy into/out of house.  Reduce air infiltration by sealing cracks  About 50% of energy loss through infiltration

Heating Degree Days:  In general, no additional heat is required if T avg > 65F.  T avg = (T high + T low )/2  Definition:Heating Degree Day (HDD) = 65-T avg  Often for some time period (like 30 days, or a year)

If T avg is 15F how many HDD? A.15 B.65 C.50 D.80

Average HDD for US.

Insulation:  The amount of insulation used in construction depends on the expected number of HDD in a given region.  Measured in terms of R value,  R is the thermal resistance. (Recall conduction discussion)

How much heat energy do we need?  Need to replace heat lost through walls, roof, etc.

Example of Heat requirements  Consider a 1000 ft 2 house in a 6000 HDD area, insulated to R-15  What heat will be lost through the walls?

Example of Heat requirements  Consider a 1000 ft 2 house in a 6000 HDD area, insulated to R-15  What heat will be lost through the walls?  Each wall is (roughly) 32 ft x 10 ft

Heat lost through walls A.115 MBtu B.2764 MBtu C.12.3 MBtu D.184 MBtu

Example of Heat requirements