In addition to heat and work flows, energy can be transferred as light, as in a photovoltaic device (solar cell). The energy content of light depends on both its wavelength (color) and its intensity. When sunlight impinges on a solar cell, some is reflected, some is absorbed and converted to electrical work, and some is absorbed and converted to heat. Consider an array of solar cells with an area of 3 m2. The power of sunlight impinging upon it is 1 kW? m?2. The array converts 17% of the incident power to electrical work, and it reflects 20% of the incident light. At steady state, what is the rate of heat removal from the solar cell array?

Question

What will be an ideal response?

Answer 1

At steady state, the net energy flow into the solar panel is zero. The rate of energy transfer to the panel as sunlight is 0.8? 1 kW/m2 ? 3 m2 = 2.4 kW (80% of the total, since 20% is reflected). The rate at which energy leaves as electrical work is 0.17? 1 kW/m2 ? 3 m2 = 0.51 kW. At steady state, the remaining energy must leave as heat. Written as an energy balance,
?Ut = 0 = 2.4 kW ? 0.51 kW + Q = 0 from which Q = ?1.89 kW. The rate of heat removal is 1.89 kW. Because solar cells inherently convert a minority of the absorbed energy into electrical work, they inevitably require removal of substantial amounts of energy as heat. This can lead them to operate at high temperature, which may further reduce their efficiency.