geothermal energy

three attributes of a high quality geothermal source are?

1. heat

2. water

3. permeability

locations with these attributes are called?

high enthalpy resources

enthalpy is defined as

• the amount of energy in a system capable of doing mechanical work

• the heat content of a substance per unit mass

• it is a function of temperature, pressure, and volume

geothermal gradient

refers to the increasing temperature at increasing depths within the earth. varies widely according to location, it is about 25 C/km in most locations but can be as much as 200 C/km

good measure of permeability

hydraulic conductivity - how easily pore fluid escapes from the compacted pore spaces

darcys law

speed of a fluid moving through a porous medium is proportional to the hydraulic pressure gradient

thermal conductivity

can be determined in the laboratory or in situ, as in a borehole or deep well, by turning on a heating element and measuring the rise in temperature with time

thermal conductivity depends on several factors

1. mineral content of the rock

2. fluid content

3. pressure

4. temperature

5. homogeneity of the rock

three essential characteristics

1. aquifer

2. cap rock

3. heat source

three temperature classifications of geothermal sources are

1. low temperature sources are <90 C. low temperature sources are useful for heating buildings, fish farms, and bathing

2. moderate temperature sources are between 90C and 150C. these sources are useful for space heating, drying, and industrial heat and in some cases are used for electricity production

3. high temperature sources are >150C. these sources are useful for generating electricity Low, moderate, and high temperature sources.

low/moderate temperature

normal ground temperature soures are useful for underground geothermal heat pumps. the ground is a constant 10C at relatively shallow depths. this is sufficient to keep heat pumps working efficiently even when the outdoor temperature is freezing

low temperature

a geothermal heat pump uses standard refrigeration cycle of operation but absorbs heat from the ground into glycol solution. can be used for space heating and cooling

cooling is accomplished with an absoprtion type refrigeration system in four steps

1. generator: heats the refrigerant

2. condenser: high pressure vapor is condensed to a liquid at higher pressure

3. expansion valve: high pressure liquid water passes through an expansion valve and becomes a cold liquid vapor mix

4. evaporator and absorber: the cold water absorbs the heat from the space to be cooled and is absorbed by the libr to form a weaker solution

heat pump efficiency

about 70 percent of the energy used by a geothermal heat pump system comes in the form of renewable energy from the ground. fossil fuel furnace may be 78-90 percent efficient, a geothermal heat pump is about 400 percent efficient

EER

energy efficiency ratio. EER values are commonly used when looking at the energy efficiency of smaller window units. the EER will not give you an accurate representation of your future energy bills, but it can provide a good way to compare two systems. EER is determined by a set of temperature and humidity

SEER

measures seasonal efficiency, which is calculated over a range of outside temperatures. more common on larger, home units. a ratio of cooling in BTU to the energy consumed in watt hours

the coefficient of performance (COP)

is the ratio of heat energy delivered to energy supplied to the heat pump. COP is unit-less, while SEER is a mix of units. geothermal heat pumps are very efficient. ground source heat pumps often a COP greater than 4. air source heat pumps tend to be less efficient, and a COP over 3 is considered good

system performance factor

a more accurate overall performance measure is the system (seasonal) performance factor (SPF/HSPF). this is the ratio of the heat produced by the heating system to the electricity consumed

higher SPF rating

indicates a more efficient system. recommended looking for heat pumps with a SPF rating between 8 and 10

system performance factor (SPF) is important because?

the yearly savings will depend on the actual cost of electricity and natural gas

depending on the source of electricity generation in ones area

when a heat pump cost more to operate than a conventional natural gas or electric heater, it may contribute less net carbon dioxide to the atmosphere than burning natural gas or heating fuel

conventional heat pumps often incluede heater coils or auxiliary heating this is because

lower temperatures may cause a heat pump to operate below the efficiency of a resistance heater

low temperature

snow melting is an application for low-temperature heat in areas with ample geothermal sources, like Iceland

what are other applications for low temperature geothermal?

greenhouses and fish farms. farming operations include aquatic plants and animals as well as certain food drying applications

high temperature

hot dry rock, water is generally injected from nearby wells. the water boils and the steam is used to drive a steam turbine

geopressuirzed reservoirs

consist of high-pressure, hot brine in deep underground reservoirs. they are characterized by dissolved methane. the best known areas for geopressurized reservoirs are along the texas and louisana gulf coast

how can energy be obtained in geopressuirzed reservoirs?

energy can be obtained from the thermal energy, from the hydraulic high pressure, or from burning the dissolved methane

hot water reservoirs

are natural reservoirs that are typically found along fault lines in the earth. the temperatures may/may not be high enough to create steam. may contained dissolved materials/acids

enhanced geothermal system (EGS)

is one in which a geothermal site is made productive by articial means including fracturing networks to increase permeability and improved drilling methods including horizontal drilling and laser drilling