Energy Efficiency
Energy efficiency quantifies how effectively a system converts energy input into useful output. It is defined as the ratio of useful energy output to total energy input, typically expressed as a percentage. Higher efficiency means less waste for the same useful output, reducing fuel consumption, operating costs, and environmental impact. The first law of thermodynamics (energy conservation) requires that energy input equals useful output plus losses. Common loss mechanisms include friction in mechanical systems, electrical resistance (I²R losses) in motors and transformers, heat transfer to surroundings in thermal systems, incomplete combustion in engines, and fluid friction (pressure drop) in piping and ducts. Efficiency definitions vary by system type. For heat engines (e.g., internal combustion engines, gas turbines, steam turbines), thermal efficiency is η_th = W_net / Q_in, where W_net is net work output and Q_in is heat input from fuel. For refrigerators and heat pumps, performance is measured by coefficient of performance (COP) rather than efficiency: COP_R = Q_L / W_in for refrigerators (cooling effect per work input), COP_HP = Q_H / W_in for heat pumps (heating effect per work input). COP values can exceed 1 because they move heat rather than convert it. The second law of thermodynamics sets an upper limit on efficiency. For a heat engine operating between temperatures T_H (hot reservoir) and T_L (cold reservoir), the maximum possible (Carnot) efficiency is η_Carnot = 1 − T_L / T_H (absolute temperatures in K or R). Real devices always have lower efficiency due to irreversibilities. Other important metrics include: specific fuel consumption (SFC = fuel mass flow rate / power output) for engines, seasonal energy efficiency ratio (SEER) for air conditioners, and annual fuel utilization efficiency (AFUE) for furnaces. Engineers improve efficiency by reducing losses (better insulation, lower-friction bearings, higher-efficiency motors), recovering waste heat (regenerators, economizers, heat exchangers), and optimizing operating conditions (part-load efficiency, variable-speed drives).
Knowledge Check
1.What does energy efficiency measure?
2.A motor consumes 10 kW of electrical power and delivers 8.5 kW of mechanical power. What is its efficiency?
3.Which of the following is a common loss mechanism in electrical motors?
4.What is the thermal efficiency of a heat engine that receives 500 kJ of heat and produces 150 kJ of net work?
5.What is the maximum possible (Carnot) efficiency for a heat engine operating between 800 K and 300 K?
6.Why can the COP of a refrigerator exceed 1, while thermal efficiency cannot exceed 1?
7.A refrigerator removes 300 kJ of heat from the cold space while consuming 100 kJ of work. What is its COP?
8.If COP_R = 3.0 for a refrigerator, what is the COP of a heat pump operating between the same temperatures?
9.An engine has a specific fuel consumption (SFC) of 0.25 kg/kWh. How much fuel does it consume to produce 80 kWh of energy?
10.Which action would most directly improve the efficiency of a steam power plant?