Calculating Heat Sink Area for 6ft Deep Heat Rejection

[Harkaran Singh]

Could anyone please help me with the area of heat sink required if I want to dump heat 6 feet below the surface?
The heat to be rejected is 20000 kW
Temperature of the fluid has to be dropped from 30 deg C to 19 deg C.
I need rough estimates of the area required to lay down looped pipelines to achieve this temperature drop

 

[BvU]

Hello Harkaran,

Not good to post the same question twice: IF someone answers, confusion results.

20 MW is a lot of heat flow to pump into the ground. And sustaining that for long periods is even more difficult. Where would the energy go ?

 

[Harkaran Singh]

Hello Harkaran,

Not good to post the same question twice: IF someone answers, confusion results.

20 MW is a lot of heat flow to pump into the ground. And sustaining that for long periods is even more difficult. Where would the energy go ?

Sorry about posting it twice.
I'm doing a study to harness geothermal energy from matured oil field. Lots of water is produced with oil and idea is to use organic rankine cycle to convert that to electricity.
The problem is that there are no good sources for cooling and I was considering dumping the heat below the surface of ground if possible.
So I wanted a rough estimate of the area required to dump this heat and weather it will be feasible

 

[jim mcnamara]

Are you positive about your numbers?

I worked through a little of this mostly to show you why some things seem way out of line:

First off:
6 feet down may not be a good choice. Does this take into account the annual change in the soil moisture line and the same depth depth of frost? The guidance we have in the US suggests that the depth of trench should be below both lowest points mentioned above by at least 0.5m. The infeed and outfeed lines in the trench should be separated by at least 1 foot. Where I work is very arid, so that level is 12-19 feet. Multiple vertical "well-like trenches" are common to minimize the impacted area and reduce trenching costs.

Also:
Your fluid may need antifreeze depending on climate and requirements like pH control. You cannot generally use any old effluent.

Next:
The length of linear trench varies by climate. At 45N latitude in North America, the guidance is 500 linear feet of trench per ton. Your value will be different. A ton is 12000 btu/hr. Your 20 mW is therefore ~5680 tons. 5680 times a minimum of (let's say) 500 ft of trench is huge. For example, that is 5680, 500 foot deep wells with pipe.

The cost of the pipe, heat exchangers, antifreeze, and digging any kind of trench is astronomical, IMO. In reality:

You definitely need specific qualified geothermal expert guidance for your location.

 

[Harkaran Singh]

Are you positive about your numbers?

I worked through a little of this mostly to show you why some things seem way out of line:

First off:
6 feet down may not be a good choice. Does this take into account the annual change in the soil moisture line and the same depth depth of frost? The guidance we have in the US suggests that the depth of trench should be below both lowest points mentioned above by at least 0.5m. The infeed and outfeed lines in the trench should be separated by at least 1 foot. Where I work is very arid, so that level is 12-19 feet. Multiple vertical "well-like trenches" are common to minimize the impacted area and reduce trenching costs.

Also:
Your fluid may need antifreeze depending on climate and requirements like pH control. You cannot generally use any old effluent.

Next:
The length of linear trench varies by climate. At 45N latitude in North America, the guidance is 500 linear feet of trench per ton. Your value will be different. A ton is 12000 btu/hr. Your 20 mW is therefore ~5680 tons. 5680 times a minimum of (let's say) 500 ft of trench is huge. For example, that is 5680, 500 foot deep wells with pipe.

The cost of the pipe, heat exchangers, antifreeze, and digging any kind of trench is astronomical, IMO. In reality:

You definitely need specific qualified geothermal expert guidance for your location.

Thank you for your insight.

Yes, I agree this is highly impractical.

My first suggestion to the operator is to use sea water for cooling but they are a bit hesitant in installing a sea water cooling system there.
Sea water is at a temp of 9 deg C.

I'm positive about my numbers. The organic fluid I'm using is R134a and the mass flowrate is 120 kg/sec.
Sseawater at 1000 kg/sec mass flowrate and 9 deg C temp I achieve the required heat dump into the sea water via a heat exchanger with 5 deg C pinch point.

Can you suggest any other methods to dump this heat? I can think of cooling towers and cooling fans but I'm not entirely sure how to quantify these two ideas.

Just to put the numbers to you:
Organic Rankine Cycle
Working fluid is R134a
Mass flowrate= 120 kg/sec
Enthalpy of the fluid before cooling is 420 kJ/kg and temp is 30.3 deg C
Enthalpy of fluid after cooling is 226 kJ/kg and temp is 19.3 deg C