geothermal systems and technologies · 5.2. working principle of geothermal heat pump 9 the higher...

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GEOTHERMAL SYSTEMS AND

TECHNOLOGIES

5. SHALLOW GEOTHERMAL SYSTEMS

5. SHALLOW GEOTHERMAL SYSTEMS (SGS)2

Shallow geothermal resources(< 400 m depth) are omnipresent.Below 15 - 20 m depth everythingis geothermal:

�the temp. field is governed byterrestrial heat flow and localground thermal conductivitystructure ± groundwater flow.

Use of low to moderate temps.

Direct use; Heating and cooling.

5. SHALLOW GEOTHERMAL SYSTEMS3

The distinction between shallow and deep

geothermal is not fixed.

In North America, shallow geothermal technology isIn North America, shallow geothermal technology is

also known under the term “geoexchange”

To use the constant, low temperatures of the ground,

there are two options:

� Ground Source Heat Pumps, GSHP,

� Underground Thermal Energy Storage, UTES.

5.1.Introduction4

GHPSs have changed the approach of geothermal energy use which until recently had

been considered as economic potential only in areas where thermal water or steam is

found concentrated at depths less than 3 km. GSHPs can be used basically everywhere

and are not as site-specific as conventional geothermal resources.and are not as site-specific as conventional geothermal resources.

GHPSs do not produce electricity, but they greatly reduce its consumption.

In winter, GHPS draw thermal energy from the shallow ground, which ranges between

10° and 21°C depending on latitude. In summer, the process is reversed to a cooling

mode, using the ground as a sink for the heat contained within the building.

Consumption of electricity is reduced by 30% to 60%, with a payback period of the

installation in 2 to 10 years.

5.1. Introduction5

Geothermal heat pumps are systems with three main components:

� the ground side to get heat out of or into the ground,� the heat pump to convert the heat to a suitable temperature level, and� the building side transferring the heat or cold into the space.

Sources, type and output of GHP

5.1. Introduction6

GSHPs are space conditioning units that use the refrigeration cycle to heat orcool a medium, using the earth as a heat source or sink.

The refrigeration cycle is reversible, so these units can be used to heat or cool.

5.2. Working principle of geothermal heat pump7

Refrigeration cycle of a CHP. WNA -heat delivery system VLH -heating supply

The most common type of heat pump is the compression heat pump (CHP).

VLH -heating supplyRLH -heating returnWQA -heat collection systemVLO -collector supplyRLO - collector return

The thermodynamic principle behind a compression

heat pump is the fact that a gas becomes warmer

when compressed into a smaller volume.


In a heat pump, the refrigerant is evaporated by the ground heat, the resultinggas is compressed and thus heated, and then the hot gas supplies its heat to theheating system.

An alternative is the absorption heat pump, where heat at higher temperature isused to drive a activate desorption-absorption cycle.An alternative is the absorption heat pump, where heat at higher temperature isused to drive a activate desorption-absorption cycle.

In both cases, the amount of external energy input (electricity or heat), has tobe kept as low as possible to make the heat pump ecologically and economicallydesirable.

The measure for this efficiency is the COP. For an electric compression heatpump, it is defined as:

inputpowerelectric

heatusefulCOP

=


The higher the COP, the lower the external energy input

compared to the useful heat. COP is dependent on:

� the heat pump itself &

� the temperature difference

between the low-temp. side and

COP versus space heating supply temperature

between the low-temp. side and

the high-temp. side.

COP can be given for the heat

pump under defined temperature

conditions, or as an average

annual COP, also called SPF.


Heat distribution system supply/

return temp.

COP1

Conventional radiators 60/50°C 2.5

Evaporator, Compressor, Condenser, Expansion valve

COP variation with temperatures

“Water-to-Air heat pump” or Conventional radiators 60/50°C 2.5

Floor or wall heating 35/30°C 4.0

Modern radiators 45/35°C 3.5

Hydronic convectors 48/38°C 3.5

1 Heat source 5 °C

“Water-to-Air heat pump” or

“Water-to-Water heat pump”.

Refrigerants with ODP=0.

R 134a, R 407C, R410A, R404A

and propane fulfill these

conditions.

5.3. Geothermal heat pump systems and application 11

GSHP offers very good conditions for achieving high COP.

A ground source heat pump system can be used not only for heating, but also for cooling.

The configurations manufactured are:

Basic schematic of water-to water GSHP system

The configurations manufactured are:� water-to-air, � water-to-water, and � water to air split type.


Direct expansion heat pumps do nothave an intermediate heat exchangeron their source side. The compressoroperation circulates the refrigerant

Refrigeration cycle–direct expansion. [33]WNA-heat delivery systemVLH-heating supplyRLH-heating return

operation circulates the refrigerantdirectly around the loop.

There is no need for a source sidecirculation pump – the compressorundertakes this role.


The water to water geothermal heat pumps areusually grouped together in a mechanical space,and can be treated as a conventional heater/chiller plant.

Basic schematic of water-to air GSHP system

chiller plant.

The unit sizes range from 3 tons to 30 tons.

The most common type of heat pump used withGSHP systems is a “water-to-air” unit ranging insize from 3.5 kW to 35 kW of cooling capacity.


Sizing the heat pump. The capacity of a heating system is defined according to themax. heat demand of a given building. The max. heat demand, called the heat load, iscalculated according to specific weather conditions and indoor air temperature.

The optimum economic size of the heat pumpdesign capacity is normally in the range of 30 toThe optimum economic size of the heat pumpdesign capacity is normally in the range of 30 to60% of the maximum heat load of the building.Such a heat pump can cover between 60 and 90%of the annual heat demand.

HP capacity and building heatrequirement (without DHW) in aheat duration diagram

5.4. Overview of ground systems for geothermal heat pump15

The ground system links the geothermal heat pump to the underground andallows for extraction of heat from the ground or injection of heat into theground.

These systems can be classified generally as:

� open, or� open, or� closed systems.

To choose the right system for a specific installation, several factors have to beconsidered:

� Geology and hydrogeology of the underground area and utilization on thesurface,

� Existence of potential heat sources like mines, and� The heating and cooling characteristics of the building(s).

5.4. Overview of ground systems for geothermal heat pump16

The various shallow geothermal systems comprise:

� horizontal ground heat exchangers 1.2 - 2.0 m depth (horizontal loops)� borehole heat exchangers 10 - 250 m depth (vertical loops)� energy piles 5 - 45 m depth� energy piles 5 - 45 m depth� ground water wells 4 - >50 m depth� water from mines and tunnels.

Systems using a heat exchanger inside the ground are called “closed” systems,while the once producing water from the ground with a heat exchanger aboveground are called “open” systems.

5.4.1. Closed vertical loop17

This systems consist of one or severalboreholes in which BHE are installed. Theboreholes may commonly be up to 200 m

Closed

vertical

loop

system

boreholes may commonly be up to 200 mdeep.

The two possible basic concepts of BHE are:

� U-pipes� Coaxial (concentric) pipes.

5.4.1. Closed vertical loop18

During the winter season the temp. of the fluid and theborehole surroundings will gradually reduce, as also theheat pump COP. In a correctly designed system the temp.will not be as low as making the heat pump to stop. This iswill not be as low as making the heat pump to stop. This isa great advantage of GSHPs compared to air as heatsource.

In the summer, these systems may provide free cooling.

Distance between the boreholes.

5.4.1. Closed loop horizontal systems19

The shallowest system.

Compared to vertical loops - less investment to

construct; somewhat less efficient due to a

lower working temperature of the fluid.

Closed horizontal ground heat exchanger

Trench ground heat exchanger

lower working temperature of the fluid.

The main thermal recharge for all horizontal

systems is provided mainly by the solar

radiation.


More compact horizontal is the so called“slinky” system.

The best efficiency of horizontal systems is

Closed horizontal-Slinky loop system

The best efficiency of horizontal systems is

obtained in fine grained types of soil with a

high content of water, such as clay and silt.

A variation of the horizontal ground source

heat pump is direct expansion system.

5.4.3. Closed loop systems submerged in surface water 22

If there is surface water available, thecheapest geothermal heat pipe system

Closed loop system submerged in surface water

cheapest geothermal heat pipe systemcould be build.

Coils should be fully soaked in water inthe depth of at least 2.4 m below thesurface.

5.4.4. Open loop systems (groundwater systems)23

Groundwater systems are more efficientthan closed loop systems. The technology“normal” groundwater wells is used forenergy extraction.energy extraction.

The temperature of groundwater ispractically constant all over the year andas such it is the best carrier of thermalenergy.

Open ground water loop system

5.5. Limitations in the application of GSHP systems24

The limitations can be physical, such as climate and geological circumstances, butmay also be connected to other site conditions.

The other potential limitations could be of a social, cultural or political nature, butmore often economical or legal.more often economical or legal.

Technical limitations For systems using the underground for seasonal storage of heat and cold, the source of energy for storage may be different:

� waste heat from industrial process cooling� waste cold from heat pump evaporators,� technical limitations such as load, duration, temperatures, availability.

Geological limitations25

The geological requirements differ according to what type of system is to be installed:

� Closed loop systems are in general applicable in all types of geology.� Open systems require a geology containing one or several aquifers.

Hydro geological limitationsHydro geological limitations

The hydro geological conditions in practice govern the design of any open loop system.

For the design and realization of such systems essential are: type of aquifer, geometry,

groundwater level and gradient, textural composition, hydraulic properties and

boundaries.

For closed loop systems these parameters are of less importance, but can in some

cases constitute limiting conditions.

Climate conditions26

Climate plays an important role in the application of GSHP systems. Oneessential condition is that the ambient temperature of the ground is reflectedby the average temperature in the air.

Another climate factor is the humidity. In hot climates with a high humidity,Another climate factor is the humidity. In hot climates with a high humidity,there will be temperature requirement for cooling that allows condensation.

Environmental limitations

GSHP energy systems will in general contribute to less global emission of

carbon dioxide and other harmful environmental substances.

5.6. Benefits of GSHP systems27

Geothermal source heat technology has several benefits, including:

� Low operating cost

� Simplicity� Simplicity

� Low maintenance

� No supplemental heat required

� Low cost integrated water heating

� No required exposed outdoor equipment

� Low environmental impact

� Level seasonal electric demand

� Longer life expectancy

geothermal systems and technologies · 5.2. working principle of geothermal heat pump 9 the higher...

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