Energy, Power, and Climate Change 8.5 Solar Power  Recall that the energy of the sun is responsible for all of our coal, oil, and wood fuels. S OLAR E NERGY  Hydroelectric dams operate using sun-lifted water.  Wind turbines use sun-driven wind currents.  In a sense, all of these energy sources are indirectly due to the sun.  When we speak of solar power it is in the direct sense, meaning energy gotten directly from the sun's rays.  The two direct solar energy devices we will discuss in this section are solar heating panels and solar cells.

Energy, Power, and Climate Change 8.5 Solar Power The sun radiates energy at a rate of 3.90  J. What is the rate at which energy from the sun reaches earth if our orbital radius is 1.5  m? S OLAR E NERGY  Since the surface area of a sphere is A = 4  r 2, we can calculate the surface area of the sphere having our orbital radius: A = 4  r 2 = 4  (1.5  ) 2 =  m 2  Then the energy per square meter per second reaching the earth (called the solar constant) is just solar constant = Power Area = 3.90  W  m 2 solar constant = 1380 W/m 2 FYI: This is approximately the rate of energy striking the earth between the tropics. At higher latitudes, the energy is spread out over a wider area, as illustrated on the next slide.

Energy, Power, and Climate Change 8.5 Solar Power S OLAR E NERGY 1380 W/m 2 FYI: Anywhere but direct-on (near the equator), the sun's intensity will be less than 1380 W/m 2. FYI: Note that the intensity will be nearly zero at the poles. This is why there are ice caps there!

Energy, Power, and Climate Change 8.5 Solar Power  The simplest means of using the sun's energy directly is through use of the solar heating panel. S OLAR H EATING P ANELS  With the solar heating panel you heat water using the sun's rays.  The hot water can be used directly from the faucet, or indirectly for central heating. i n s u l a t i o n black absorber water pipe glass cold water in hot water out

Energy, Power, and Climate Change 8.5 Solar Power A 6 m 2 solar heating panel is located at a place where the intensity of the sun is I = 958 W/m 2. S OLAR H EATING P ANELS (a) What is the incident power on the panel? P = IA= (958)(6) = 5748 W (b) If the efficiency of the panel is 42%, how much energy is absorbed per second by the water? P useable = 0.42P = 0.42(5748) = 2414 W FYI: This is 2414 J / s. (c) If 2 liters (2 kg) of water pass through the panel each minute, by how much will its temperature change? Hint: c = 4200 J/kg  C. Q = P useable t = 2414(60) = J Q = mc  t = 2(4200)  t  t = 17  Question: If the water flow rate is slowed down (or speeded up), will the change in temperature be the same?

Energy, Power, and Climate Change 8.5 Solar Power  The solar cell (photovoltaic cell) converts the photons in sunlight directly into electricity. S OLAR C ELL (PHOTOVOLTAIC CELL)  When the photons are absorbed by the semiconductor material of the solar cell, electrons are released.  The potential difference across a photovoltaic cell is small. If many are placed in series, you increase the voltage; if placed in parallel, you increase the current.

Energy, Power, and Climate Change 8.5 Solar Power A photovoltaic cell has an area of 1.00 cm 2 and an efficiency of 12.5%. S OLAR C ELL (PHOTOVOLTAIC CELL) (a) If the cell is placed in a position where the sun's intensity is I = 1250 W/m 2, what is the power output of the cell? A = 1 cm 2 1 m 100 cm 2 = m 2 P useable = 0.125IA = 0.125(1250)(0.0001) = W (b) If the cell is rated at V, what is its current output? P = IV I = P/V= /.5 = A (31.3 mA) (c) If 10 of these cells are placed in series, find the current and voltage outputs:  When placed in series, voltage increases, current stays the same: I = A, V = 10(.5) = 5.00 V. (c) If 10 of these cells are placed in series, find the current and voltage outputs:  When placed in parallel, current increases, voltage stays the same: V = V, I = 10(.0313) = A.

Energy, Power, and Climate Change 8.5 Solar Power Draw a Sankey diagram for the photovoltaic cell. S OLAR C ELL (PHOTOVOLTAIC CELL) Energy in incident sunlight Wasted Heat Useable electricity FYI: Just as we included the energy needed for fuel production in the Sankey diagram for a reactor, we could include the energy needed to manufacture the solar cells. This could potentially nullify the benefits of a solar cell!