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Energy Flows and Balances

Energy Flows and Balances. Units of Measure. BTU – amount of energy required to heat one pound of water, one degree Fahrenheit. Calorie – amount of energy required to heat 1 ml water 1 degree Celsius. Energy Balances and Conversion.

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Energy Flows and Balances

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  1. Energy Flows and Balances

  2. Units of Measure BTU – amount of energy required to heat one pound of water, one degree Fahrenheit Calorie – amount of energy required to heat 1 ml water 1 degree Celsius

  3. Energy Balances and Conversion The form of the available energy is often not the form that is the most useful so it is common to have to convert one form of energy to another. Water in lake – turbine – electricity – light and heat Bio-mass – combustion – steam - generator - electricity Wind – windmill – generator - electricity Conversion always less than 100% efficient

  4. 0 Rate of Energy accumulated Rate of Energy In Rate of Energy Out Rate of Energy Generated = - + Rate of Energy Out Rate of Energy Wasted Rate of Energy In Useful energy Out = - X 100 Energy In Since energy “flows” can use the same concepts as materials balance to analyze At Steady-State Efficiency:

  5. Bomb Calorimeter

  6. Calorimeter Example A calorimeter holds 4 liters of water. When a 10 gram sample of a waste-derived fuel is combusted the result is a 12.5o C rise in temperature. What is the energy value of the fuel? Energy In = Energy Out (The idea behind a calorimeter is that no energy is wasted. It is all captured in the device.) Energy Out = 12.5o C x 4 L x 103 ml/L x 1 g/ml = 50 x 103(0C). G, or calories = 50 x 103 calories x 4.18 (J/cal) = 209 x 103 J Energy In = 209 x 103 J Energy Value of the Fuel = (209 x 103 J/g)/ (10 g) = 20,900 J/g

  7. Heat Energy Mass of Material Absolute Temperature Of the Material = X Heat Energy This is only true when the heat capacity of the material is independent of temperature. In particular when a phase change occurs this is not true. (Water to Steam) Energy Balance at Steady-State with two inflows 0 = Heat Energy In - Heat Energy Out + 0 0 = [T1Q1 + T2Q2] - T3Q3 T3 = [T1Q1 + T2Q2] / Q3 Also: Q3 = Q1 + Q2

  8. Example A coal-fired power plant discharges 3 m3/sec of cooling water at 80o C into a river that has a flow of 15 m3/sec and a temperature of 20o C. What will the temperature of the river be immediately downstream of the discharge? T3 = [T1Q1 + T2Q2] / Q3 = [(80 + 273)(3) + (20 +273)(15)] / (3 + 15) = 303oK = 30o C

  9. Energy Sources and Availability Non-Renewable Sources Nuclear Power Coal, Peat, and Similar Products Oil Natural Gas Renewable Sources Hydropower from Rivers Hydropower from Tides Wood and Other Bio-mass Solar Power Refuse and other Wastes Wind

  10. What responsibility do we have to future generations?

  11. Energy Equivalence Arithmetic Energy Equivalence – based on energy amounts only Conversion Energy Equivalence – takes into account the energy loss in conversion For Example If gasoline has an energy value of 20,000 BTU/lb and refuse- derived fuel has an energy value of 5,000 BTU/lb, the arithmetic energy equivalence is: 20,000/5,000 = 4 lb refuse / 1 lb gasoline It has been estimated that 50% of the energy in refuse derived fuel is required for processing, therefore, the actual net energy in the refuse is 2,500 BTU/lb. So: Conversion energy equivalence = 20,000 / 2,500 = 8 lb refuse / 1 lb gasoline

  12. Electric Power Production Present power plans are less than 40% efficient

  13. 0, S.S. Rate of Energy accumulated Rate of Energy In Rate of Useful Energy Out Rate of Wasted Energy Out = - - Simplified: Heat Engine Energy Balance 0 = Qo - QU - QW Efficiency (%) = QU/Qo

  14. The most efficient engine possible is called a Carnot Engine. Its efficiency is calculated as: EC(%) = (T1 – To)/T1 x 100 Where: T1 = absolute temperature of the boiler T2 = absolute temperature of the cooling water Since this is the best possible: (QU/Qo) < (T1 – To)/T1 Typical conditions for a power plant are: T1 = 600 + 273 = 873, and T0 = 20 + 273 = 293 EC = (873 – 293) / 873 = 66% Because real power plants have many other types of energy losses (heat in stack gases, evaporation, friction) their actual efficiency is about 40%. This figure is confirmed by operational data.

  15. Where does all of this energy go? All of it is dissipated in some way into the environment 60% of the energy content of the fuel that comes into the plant is released to the environment as heat 15% stack gases, 45% cooling water Thermal pollution

  16. Cooling this water before discharge is a significant problem Cooling towers such as these can add up to 250% to the cost of a nuclear power plant Why is it better to allow this heat to be discharged to the atmosphere rather than to a water body? What else could you do with this heat?

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