TIPS ON SOLAR HEATING
WHAT IS SOLAR HEATING?
The amount of solar energy (insolation) that comes into a building through the windows (size, orientation, glazing, and shading) depends on time of year, location, and percentage sunshine (clouds and haze).
The solar heating can be estimated from tables of clear-day insolation by latitude and percentage sunshine by month. Average insolation values by month that take into account cloud cover for different angles of surfaces are available on the Internet.
The data for vertical insolation indicate that in most locations vertical windows on the south can be net heat gainers, depending on climate and R value of the windows. Surprisingly, even single-pane windows on the south can be net energy gainers in temperate climates.
Also, during the summer vertical windows let in less solar energy due to the angle of the sun (cosine factor). There is more reflection due to the larger angle of incidence.
Example 5.1
Calculate the amount of solar heat that comes through a south-facing window (single pane) for January (70% sunshine).
Vertical window, 1.2 by 2.5 m, single pane, transmission = 90%
Area = 3 m2, insolation for January is 6 kWh/m2 per clear day
Energy hitting window per day = 3 m2 * 6 kWh/m2 = 18 kWh/day
Energy transmitted = 0.9 * 18 kWh/day = 16 kWh/day = 55,000 Btu/day
Energy for month = 16 kWh/day * 31 days * 0.70 = 350 kWh = 1.2 * 106 Btu
Maps of solar insolation for the United States by month are available from the National Renewable Energy Laboratory (NREL) for different types of collectors and orientation (http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/). The two-axis concentrating collector gives the normal to the sun. The maps also take into account the percentage sunshine, so the average values of energy per day do not have to be adjusted.
Thursday, December 25, 2014
ENERGY PRODUCTION OF OUR SUN BASIC INFORMATION
How much energy is produced in the sun?
The energy produced by nuclear reactions in the interior of the Sun must equal the amount of energy radiated from the surface, since otherwise the Sun could not have been structurally stable over long periods of time.
Evidence for the stability of the Sun comes from several sources. Stability over a period of nearly 3×10^9 years is implied by the relative stability of the temperature at the Earth’s surface (oxidised sediments and fossil remains indicate that water in its fluid phase has been present throughout such periods).
Stability over an even longer time is implicit in our understanding of the evolution of the Sun and other similar stars. As an indication of this stability, the variations in the radius of the Sun believed to have taken place since its assumed formation from clouds of dust and gas.
The conversion of energy contained in the atomic constituents of mainsequence stars such as the Sun from heat of nuclear reactions (which transforms hydrogen into helium) to radiation escaping from the surface is largely understood.
The basis for regarding such radiation as a renewable source is that it may continue essentially unaltered for billions of years. Yet there is also a possibility of tiny variations in solar energy production that may have profound implications for life on the planets encircling it.
The energy produced by nuclear reactions in the interior of the Sun must equal the amount of energy radiated from the surface, since otherwise the Sun could not have been structurally stable over long periods of time.
Evidence for the stability of the Sun comes from several sources. Stability over a period of nearly 3×10^9 years is implied by the relative stability of the temperature at the Earth’s surface (oxidised sediments and fossil remains indicate that water in its fluid phase has been present throughout such periods).
Stability over an even longer time is implicit in our understanding of the evolution of the Sun and other similar stars. As an indication of this stability, the variations in the radius of the Sun believed to have taken place since its assumed formation from clouds of dust and gas.
The conversion of energy contained in the atomic constituents of mainsequence stars such as the Sun from heat of nuclear reactions (which transforms hydrogen into helium) to radiation escaping from the surface is largely understood.
The basis for regarding such radiation as a renewable source is that it may continue essentially unaltered for billions of years. Yet there is also a possibility of tiny variations in solar energy production that may have profound implications for life on the planets encircling it.
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