Geothermal & GSHP

Question 1

Is geothermal renewable ?

Answer 1

Geothermal energy extraction may be more like ‘mining’ for heat, when the heat is taken out it takes a very long time for the well to recover to its original thermal characteristic. (Italy and Devon we’re still temperatures drop because more heat is being taken out)

Question 2

Explain how geothermal works and the difference between using ‘hot rocks’ and aquifers (3)

Answer 2

• Geothermal power is the term given to harvesting thermal heat from the ground. It is similar to
  ‘mining’ of other resources, other than it is heat rather than a material which is removed from the ground.

-Aquifer heat sources are likely to be more resilient to thermal harvesting as the natural circulation of subterrain water is able to shift heat around.

• Dry rocks is similar to fracking, heat is contained in the solid rocks (the rocks are hotter than their environment due to natural radioactive decay). The rocks are shattered to introduce small percolation channels and water is forced through the rocks
  .

Question 3

What are the main difference between using geothermal processes to get low grade heat and power generation? (3)

Answer 3

• both pump cool water into the geological formation and extracting much hotter water from another area of the formation.
• For thermal use:relatively low grade heat (Southampton), Equipment can be more ‘agricultural’.
• For power generation: need reasonably hot heat source, say 150C , otherwise the thermal efficiency will be uneconomically low

Question 4

Why is geothermal power generation limited to certain areas whilst GSHP can go anywhere?

Answer 4

• high grade heat we need plus 150, therefore need specific areas with hot rocks (radioactive decay)
• GSHP use reverse heat engines to ‘transform’ large quantities of low grade heat into smaller quantities of higher grade heat. This means that the geographical location where GSHP can be used become (almost) limitless

Question 5

What are some of the desired characteristic of the pipe-in-pipe system for geothermal?

Answer 5

• very high heat transfer in vertical locations. we use water to achieve this which provides good thermal coupling between the heat and the user
• thermal decoupling between piper inside diameter and the outside: don’t want to waste the heat we’ve harvested

Question 6

What is meant by a short carbon cycle?

Answer 6

Short carbon cycle in an energy/power generation context refers to the sequestration of CO2 from the air by biomass over time scales measured in years, followed by the release of the chemical energy bound up in the biomass in much shorter time scales. The carbon being returned to the environment, where it can be re-sequestrated and the cycle is repeated. Short carbon cycles are usually regarded as renewable, sustainable and climate friendly in that the carbon ‘dumped’ into the environment has been taken from the environment in timescales short enough to for the ecosystem to smooth out the perturbations.

Question 7

What’s the difference between a ground heat pump and geothermal?

Answer 7

Geothermal energy harvesting refers to the removal of thermal energy from the ground in regions where its temperature is well above ambient. Heat is allowed to flow down temperature gradients. Ground source heat pumps take heat from the ground even though the ground’s temperature may be below the temperature required. The heat pumps are reverse heat engines; they are used to ‘pump’ out heat from the ground.

Question 8

In terms of carbon footprint, discuss whether the use of GSHP rather than burning gas is beneficial or not.

Answer 8

Using the soil to act as a large thermal store which is seasonal warmed and cooled, improves the thermal performance of the system greatly. It reduces the ‘lift temperatures’ required and ensures that the bore hole does not progressively get colder and colder with years of use. A fully balance suytem becomes sustainable with no deterioration in thermal performance with age.

Question 9

How will the efficiency of GSHP vary throughout the year?

Answer 9

The thermal efficiency of the GSHP installation will be very high when the temperature ‘lift’ required is low. Hence it will be highest at the start of the seasons when heat is just beginning to be withdrawn or when heat is just beginning to be deposited.

Question 10

What does that Carnot efficiency represent for a GSHP? also state the equation

Answer 10

heat energy out for the electricity put in to run the system

Cop = Th/(Th-Tc)