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Week 12

Conservation of EnergyEnergy Cost of Agriculture

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Insulation• About 12 % of conductive heat losses occur through the

ceiling and 17 % occurs through the walls of a house• The R-value is the figure used to indicate the effectiveness of

a insulation material i.e. the resistance conduction• The R value is measured in m2·K / W or m2·°C / W • The R value is given for a specific thickness. E.g. R value of

2.3 m2·K / W for 100 mm thick mineral wool. Alternatively it is given as 22.3 m·K / W without giving the thickness.

• Rate of heat transfer = A x (TH – TL) / RA: Area in m2

T: Temperatures in °C or KelvinR: R value in m2·K / W

• Sometimes the U value for insulation materials are supplied: U = 1/R

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Insulation• E.g. The outside walls of a house measures 10 x 5 x 3 m• The average indoor and outdoor temperatures are 25 °C and 10 °C

respectively

• The R value of the brickwork is = 0.8 m2·K / W • What is the heat loss through the walls?• Q = A (TH – TL) / R

= 2 x (10 + 5) x 3 x (25 – 10) / 0.8Q = 1.69 kW

• If insulation with a R value of 2.5 is added, what would be the heat loss?• Q = A (TH – TL) / R

= 2 x (10 + 5) x 3 x (25 – 10) / (0.8 + 2.5)Q = 0.41 kW

• R values add up (like resistors in series) when more than one layer with different R values are used

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Typical SA Insulation

Thickness Price/m2 R-Value

Think Pink Aerolite 100 mm R 15.63 2.5

50 mm R 6.25 1.25

Isotherm 100 mm R 23.94 2.2

50 mm R 11.97 1.1

ThermocousTex 50 mm R 44.99 1.35

Note that using 2 plies of Isotherm will give you an R value of 4.4 for R 47.88. This seems to be a much better deal than paying R44.99 for 50 mm ThermocousTex with a R value of only 1.35? This is not always the case, there might only be limit space for the insulation and then the 50 mm option are more viable.

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Germany

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Solar Energy

• The effect of window orientation and has a large influence on the indoor heat gain

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Heat Pumps

• The ground temperature 1 m below the surface is almost constant throughout the year.

• In winter, this temperature could be higher than the ambient air temperature

• A heat pump uses the ground as a heat source or “stepping stone” for heating the inside of a house.

• This is more economic than directly converting electricity into heat using a conventional electric heater

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Heat Pumps

• One unit of electricity can give 3 units of heating

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9.7 Commercial and Industrial Energy Conservation

• Commercial buildings has a much higher standard for indoor air quality and light intensity in comparison to residential buildings

• The heat load in commercial buildings are due to lights, people (on average 100 W or more per person) and electrical equipment

• District cooling and district heating become viable options for commercial applications

• District cooling is more economic due to economies of scale. If night time and thermal storage is used, the plant size and running cost can be significantly reduced

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Cogeneration and district heating

• Cogeneration (electricity and heat) is a viable option in many areas

• District heating can also be fired by biomass• E.g. of a small town close to Munich in Germany. A new

housing development uses a district heating system. The connection fee is E8000 vs the conventional E15000 installation cost for an oil burner.

• The heating cost is 50% of the conventional heating cost• Heating is done by burning agricultural waste in a central

heating system. The process is CO2 zero and more economic than the conventional oil systems.

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Conservation in Industry

• About half the electricity in the world is used to run electric motors• The majority of electric motors are constant speed motors. The

rotate at 3600 rev/min• When a constant speed motor is used to power a pump, the flow

rate is mostly done by valves. This wastes a lot of energy. Instead a variable-speed drive is the solution

• New more efficient processes should replace existing processes e.g. by forming steel product to the desired shape rather than reheating it later again

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East German Ampellman

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LED Replacements

Seattle BridgeLED traffic lights

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LED – Light emitting diodes

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Energy use in Agriculture

• First people were hunters

• Later they settled down and became farmers

• Later these farmers could support other people by their surplus

• This lead to the enhanced development of cultures

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24.1 Traditional Agriculture

• E.g. of the Tsembaga people in New Guinea• Swidden Agriculture (Slash and burn) is practices• A section of forest is cut and the remainder burned. • Human power is employed for the burning, erecting fences, planting,

weeding, harvesting and transportation of the produce.• Total energy input per hectare is estimated to be 5.8 GJ• Total energy output is 100 GJ/ha of which about 62 GJ/ha is used for

human consumption, the remainder for animal feed• Humans need 6 – 10 MJ/day (2.1 - 3.6 GJ/year)• One hectare of Tsembaga farming can support 17 of their people for a

year.• The population of 200 Tsembaga people use about 35 hectares of

fields.• This is small enough to allow the forest to grow back and therefore

allow for sustainable living!

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24.2 Agriculture in North America• Humans worldwide use 25 – 40 % of all biological material produced on

Earth• The use of people for agriculture has decreased over the years• In the US, the 1900 style of agriculture leads to 75 % of the population

involved. Today less than 5 % of the US populations is working in the agricultural field

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Machines replace human labor• Energy efficiency varies for different kinds of production• Primitive agricultural methods are much more energy-

efficient than modern agricultural methods

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Machines replace human labor• Figure 24.3 illustrates the

decrease in human labor over the past few decades

• The decreases are due to the increase utilization of other energy forms like machinery for harvesting and mechanical systems for feeding animals

• Crops where a high portion of human care is required shows the lowest decrease in human labor e.g. tobacco

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• The increased utilization of energy leads to increased yields

Machines replace human labor

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Pesticides and Fertilizers• The use of pesticides has lead to increased yields• Pesticides have side effects• Alternatives are relying on natural enemies, rotating of crops, applying

pesticide sparingly at the optimal time, etc• In 1970 about 642 MJ (3212 kWh) per hectare was attributed to

artificial fertilizer use.

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Pesticides and Fertilizers• The use of fertilizer has increased over the years• Nitrogen is the major fertilizer and in mainly use in the form of ammonia

• Ammonia is produced from natural gas (methane CH4). This process is very energy intensive

• Animal manure or rotating crops is a possible solution to get around without using fertilizers.

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24.3 Energy efficiency of the agricultural system

• In the US, farms consume 4% of the total energy supply. If the entire food systems is considered, this figure becomes 17 %

• The figure shows all the energy role players in the food system

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24.3 Energy efficiency of the agricultural system

• The production of energy attributed to food has kept pace with population growth

• Energy use in the food industry has far outstripped the population growth

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Monoculture vs Diversity

• The most stable natural ecosystem seem to be the most complex and to have the lowest apparent productivity (from a human perspective)

• This is because in nature everything depends of feeds on everything else

• Monocultures are simplified ecosystems and have high productivity• These ecosystems are fragile due to the lack of complexity• Nature : maximize stability by complexity. Productivity is total

biomass produced• Humans : maximize productivity by simplicity. Productivity is

biomass that can be harvested