Hydrologie(elias jamhour)

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Lebanese University.

Faculty of Engineering.

Department-1-

Hydro-thermal energy.Heat transfer system.

By: Elias Jamhour

1/31/2010


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Overview on Traditional systems:

Traditional water-source heat pumps use a fossil-fuel-fired boiler as a heat source

during the winter and an evaporative cooling tower to reject heat during the

summer. This is sometimes referred to as a boiler/tower system; The water loop

temperature is maintained between 60 and 90F. When the loop temperature falls

below 60F, the boiler adds heat, and when the loop temperature exceeds 90F, the

cooling tower rejects heat.

How the Geothermal system works?

The earth has the ability to absorb and store heat energy. To use that stored energy,

heat is extracted from the earth through a liquid medium and is pumped to the heat

pump heat exchanger. There, the heat is used to heat buildings and homes.


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And reversely, in summer the process is reversed and indoor heat is extracted from

the building and transferred to the earth through the liquid.

Advantage of the system:

A GHP heat pump system is a water-loop system where the boiler and cooling

tower have been replaced with a buried earth heat exchanger (ground loops).

Heat is absorbed from or rejected to the ground, and the external energy needed to

operate a boiler or cooling tower is eliminated.

we should note that: heat pumps must work harder to move heat up a steeper

temperature gradient, and if the gradient is too steep a heat pump will not work at

all.


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As we know, during the times of the year when heating and cooling are required,

ground temperatures are nearer room temperature than outdoor air temperatures.

Thus during the summer cooling season, the ground is cooler than the air, and the

ground loop is able to condense the refrigerant at lower temperatures which meansthat less compressor power is needed to pressurize the refrigerant vapor.

Ground-source(geothermal)heat pump in cooling mode.

Likewise during the winter heating season, the ground is warmer than the air, and

the ground loop is able to evaporate the refrigerant at higher temperatures, whichagain means that less compressor power is needed, since the pressure drop through

the expansion valve can be less and still vaporize the refrigerant liquid.


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Ground-source (geothermal) heat pump in heating mode.

As a conclusion, geothermal heat pumps are 3.5 5 times as efficient as the most

efficient fossil fuel furnace. Instead of burning a combustible fuel to make heat,

they simply move heat that already exists. By doing so, they provide 3.5 5 units

of energy for every unit used to power the heat-pump system.


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Principal parts of the system:

The water/refrigerant heat exchanger, refrigerant/air heat exchanger, and all

refrigerant loop components are contained in a single enclosure, which can be

mounted horizontally (above a dropped ceiling), vertically (in a utility closet), or as

a console (against the wall or under a window). Maintenance experience with GHP

systems suggests that horizontal units should NOT be mounted above the dropped

ceiling in classrooms, because this makes access difficult, adding considerable

time (and labor cost) to routine servicing of the heat pump units. Vertical and

console units are easier to service and should be used if possible. When horizontal

units must be used, they should be mounted above hallways adjacent to the rooms

they serve.


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Horizontal GHP unit installation above Vertical GHP unit installation in a mechanical utility

a dropped ceiling.


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Console GHP unit.

GHP refrigerant fluids

The refrigerant working fluid used in geothermal heat pumps is R22, which also is

the most popular refrigerant for packaged air conditioners and air-source heat

pumps and which was made as a substitute for R12. It can produce more cooling

capacity from the same size compressor with no significant power penalty.

Because R-22 contains some chlorine, it is classified as an HCFC (hydro-chloro-

fluoro-carbon).

The ozone depletion potential of R22 is only 5.5% compared with the depletion

potential of R11 and R12 (CFC-11 and CFC-12), which are the two refrigerants

thought to be most responsible for atmospheric ozone layer depletion.

A long-term substitute for R22 is R410a, a chlorine-free blend of two HFC

refrigerants (R32 and R125), which is safe and easy to use, with no ozone-depletion potential. R410a has up to 6% greater refrigeration capacity than R22

and its Energy Efficiency Ratio (EER) is 5-6% higher, offering improved

performance in addition to its environmental benefits.

But its operating pressures are nearly 50% higher than R22, requiring a redesign of

the compressor as well as other components. In addition, there are compatibility


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issues with the lubricants, cleaners and other fluids used in the heat pump

manufacturing process.

Ground loups:

An open-loop system that consumes water from a well.

A closed-loop system continuously circulates its heat transferring solution in

pressurized pipe.The term closed-loop is used to describe a geothermal heat

pump system that uses a continuous loop of special buried plastic pipe as a heat

exchanger. The pipe is connected to the indoor heat pump to form a sealed,

underground loop through which water or an anti-freeze solution is circulated.

Several forms of closed loops are figuring in this picture:

Trenches are normally four to six feet deep [0.5 1.8 meters]. One of the

advantages of a horizontal loop system is being able to lay the trenches according

to the shape of the land. As a rule of thumb, 125-300 feet of trench are required

per ton of heat pump capacity [11-27 meters per kW of capacity].


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1 to 6 pipes pre trench may be used, depending upon the optimal design for the

yard. More pipes per trench shorten the total amount of trench required.

Closed loop systems can also be vertical. Holes are bored to about 150 300 feet

per ton of heat pump capacity [13 27 meters per kW of capacity]. U-shaped

loops of pipe are inserted in the holes. The holes are then back-filled with a sealing

solution (grouting material).

Piping material:

Closed-loop systems should only be installed using the appropriate high-density

polyethylene pipe. Properly installed these pipes will last over 50 years. They are

inert to chemicals normally found in soil and have good heat conducting

properties. PVC pipe should not be used under any circumstances.

The only acceptable method to connect pipe sections is by thermal fusion. Pipe

connections are heated and fused together to form a joint stronger than the original

pipe. Mechanical joining of underground pipe for an earth loop is never an

accepted practice. The use of barbed fittings, clamps and glued joints is certain to

result in loop failure due to leaks.

References:

http://www.geo4va.vt.edu/A3/A3.htm
http://www.geo4va.vt.edu/A3/A3.htmhttp://www.geo4va.vt.edu/A3/A3.htm

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