Volume 7 geothermal energy 7 06 – shallow systems geothermal heat pumps

Volume 7 geothermal energy 7 06 – shallow systems geothermal heat pumps Volume 7 geothermal energy 7 06 – shallow systems geothermal heat pumps Volume 7 geothermal energy 7 06 – shallow systems geothermal heat pumps Volume 7 geothermal energy 7 06 – shallow systems geothermal heat pumps Volume 7 geothermal energy 7 06 – shallow systems geothermal heat pumps Volume 7 geothermal energy 7 06 – shallow systems geothermal heat pumps

7.06.1 Introduction

References

Relevant Websites

7.06.1 Introduction

A new chapter in geothermal direct use opened with the advent of geothermal heat pumps (GHPs) This technology enables space heating, cooling, and domestic warm water production with the same installation The GHP application is now in the focus of private, public, and municipal interest [1]

GHPs are one of the fastest growing applications of renewable energy in the world and definitely the fastest growing segment in geothermal technology in an increasing number of countries Recent statistical data [2] indicate rapid growth (see Figures 1 and 2)

GHPs represent a rather new but already well-established technology, utilizing the immense renewable storage capacity of the ground GHPs use the relatively constant temperature of the ground to provide space heating, cooling, and domestic hot water for homes, schools, factories, and public and commercial buildings The applicational size can vary from single-family homes with

1–2 borehole heat exchangers (BHEs) to large-scale complexes with hundreds of BHEs The decentralized systems can be tailor-made, taking into account the local conditions It is essential to employ proper installation design that takes into account meteorological conditions, ground property, and technical supply conditions By these means, reliable long-term operation can

be secured Of the local conditions, the thermal conductivity of ground materials and the groundwater properties are of key importance

7.06.2 The Resource

Shallow geothermal resources (the heat content of rocks in the top few 100 m of the continental crust) represent a major and ubiquitous energy source The Earth as planet can afford to give off heat by a thermal power of 40 million MW, without cooling down Without utilization, the terrestrial heat flow is lost to the atmosphere In this case, the isotherms run parallel to the Earth’s surface (i.e., horizontal in flat terrain) and the heat flow lines are perpendicular to them If, instead, the heat flow can be captured, for example, by a heat extraction device such as a BHE (see later), the isotherms are deformed and the heat flow lines can be diverted toward heat sinks (Figure 3)

Isotherms

Heat sink Terrestrial

heat flow

Heat production

atmosphere

30 000

25 000

t20 000

15 384

15 000

10 000

1854

0

2 50 000

2 00 149

50 000

0

Figure 1 Worldwide installed capacity (MWt) of geothermal heat pumps Data from Lund JW, Freeston DH, and Boyd TL (2010) Direct utilization of geothermal energy 2010 worldwide review Proceedings of the World Geothermal Congress 2010 Bali, Indonesia (CD-ROM) [2]

Figure 2 Worldwide use (TJ yr−1) for heating by geothermal heat pumps Data from Lund JW, Freeston DH, and Boyd TL (2010) Direct utilization of geothermal energy 2010 worldwide review Proceedings of the World Geothermal Congress 2010 Bali, Indonesia (CD-ROM) [2]

Figure 3 Principle of geothermal heat extraction and production Reproduced from Rybach L (2008) The international status, development, and future prospects of geothermal energy Proceedings of Renewable Energy 2008 Busan, S Korea (CD-ROM) [3]

342 W m–2

Emitted by atmosphere

Absorbed by atmosphere

Incoming and outgoing heat fluxes

Latent heat 78

24

67

235 W m–2

40 Atmospheric window Greenhouse gases

324 backradiation

324 Absorbed by surface

390 Surface radiation

78 Evapotranspiration

168 Absorbed by surface

Reflected by surface

30

77

77

Reflected by clouds,

aerosol and atmosphere

24 Thermals

Figure 4 Incoming and outgoing solar radiation at the Earth’s surface Reproduced with modification from Kiehl JT and Trenberth KE (1997) Earth’s annual global mean energy budget Bulletin of the American Meteorological Society 78: 197–208 [4]

~15 m

400 m

Mixed resources

Shallow geothermal resources

Geothermal heat flow

Temperature (� C)

T (z) z(N.z.)

Figure 5 The depth domain of shallow geothermal resources (left) and the general temperature–depth function In the topmost 10–20 m, the annual surface temperature variations are noticeable z (N.Z.) denotes the depth of the neutral zone For details, see text

Occasionally, it is claimed that the shallow geothermal resource consists of stored solar energy This is completely wrong – the thermal conditions at the Earth’s surface are balanced: the solar heat energy irradiated is reradiated completely back to the atmosphere (otherwise the Earth’s surface would be heated up and life would be impossible) Figure 4 shows the numerical values

of solar incoming and outgoing heat fluxes

By definition, geothermal energy is the energy in the form of heat beneath the surface of the solid Earth The domain of shallow geothermal energy is customarily considered to comprise the topmost 400 m of the Earth’s continental crust Temperature changes at the earth’s surface propagate down to a certain depth; at any location, there is a depth at which the amplitude of annual variations decreases to become negligible This depth is termed the ‘depth of neutral zone’, z(N.Z.); Figure 5

thermal conductivity λ For example, in moderate climate, the depth z(N.Z.) is approximately 10λ1/2

; Figure 6 depicts the dependence of z(N.Z.) on λ

Depth (m)

25

20

15

10

5

0

Ground thermal conductivity λ (W m−1 K −1 )

Figure 6 Depth of the neutral zone, z (N.Z.), as a function of ground thermal conductivity For details, see text

7.06.3 Geothermal Heat Pumps

A GHP is a decentral heating and/or cooling system that moves heat to or from the ground It uses the Earth as a heat source (in the winter) or a heat sink (in the summer) GHPs are also known by a variety of other names, including ground-source, geoexchange, earth-coupled, earth energy, or water-source heat pumps (HPs) They can be designed and installed in sizes from a few thermal kW

to several MW capacity (the latter in modular assemblage)

7.06.3.1 Common Types

There exist mainly two types of GHPs: closed and open (Figure 7) In ground-coupled systems, a ‘closed loop’ of plastic

50–300 m depth A water–antifreeze solution is circulated through the pipe Thus heat is collected from the ground in the winter and optionally heat is rejected to the ground in the summer An ‘open-loop’ system uses groundwater or lake water directly as a heat source in a heat exchanger and then discharges it into another well, a stream, or lake, or even on the ground The installation of horizontal coils requires relatively large surface area and extensive earthworks (digging the ground down to the level of coil layout); the prerequisite for extracting the heat of groundwater is the presence of a shallow water table For these reasons, the most widespread technology of shallow heat extraction is by BHEs Heat extraction is established by closed-circuit fluid circulation (a few m3 h−1 of pure water or with an antifreeze additive) through the BHE and the evaporator side of the HP

Three basic components make up a GHP system: (1) the heat extraction/storage part in the ground; (2) the central HP; and (3) the heat distributor/collector in the building (e.g., floor panel) These three circuits are shown in Figure 8 The key component is the

Vertical

BHE

HP Horizontal

Two well

Figure 7 Closed-loop (vertical and horizontal) and open-loop (groundwater) heat pump systems The green arrow indicates the most common system, with borehole heat exchangers (BHEs) The heat pump (HP) is shown in red Reproduced with modification from Lund J, Sanner B, Rybach L, et al (2003) Ground source heat pumps – A world review Renewable Energy World July–August: 218–227 [5]

Connection

exchanger

Heat exchanger pipes

Supply system Heat pump circle

Backfill (grouting) Borehole

heat

Figure 8 The three main circuits of a geothermal heat pump system: (1) the heat source (in this case, a BHE); (2) the heat pump; (3) the heating/cooling circle (in this case, floor panel heating)

Heat pump

Single or double U-tube

Floor panel heating (35 � C)

Backfilled borehole,

10 cm diameter

Borehole depth

50 − 300 m

Figure 9 Sketch of a geothermal heat pump system with a single borehole heat exchanger Colored arrows indicate circulation in the U-tube heat exchanger and black arrows heat extraction from the ground (heating mode in winter) In summer, the arrows are reversed; heat is extracted from the building and stored in the ground

HP In essence, HPs are nothing more than refrigeration units that can be reversed In the heating mode, the efficiency is described

by the coefficient of performance (COP), which is the heat output divided by the electrical energy input for the HP Typically, this value lies between 3 and 4 [6] Except for larger singular applications where gas-driven HPs are used, most HPs use electricity for their compressors Therefore, GHPs are electricity consumers The source of electricity varies from country to country; it would be elegant if the electricity to operate the GHPs would originate from renewable sources like solar, wind, or even geothermal The

7.06.3.2 Further Types: Energy Piles, Geothermal Baskets

‘Energy piles’ are foundation piles equipped with heat exchanger piping The piles are installed in ground with poor load-bearing properties The energy piles use the ground beneath buildings as heat source or sink, according to the season The systems need careful design, taking into account especially the spacing between the piles, the ground thermal properties, and possible static influence of temperature changes in the piles Figure 10 shows installation and system sketch of energy piles

Figure 10 Energy pile system sketch (left) 1: energy piles; 2: connections; 3: distributor; 4: general connection; 5: central unit Installation of heat exchanger pipes in a foundation pile (right)

Figure 11 Geothermal basket example and placement sketch: a basket with 0.5 m diameter and 2 m length (total pipe length 55 m, left); implantation in 1.5–3.5 m deep holes (right)

‘Geothermal baskets’ are spirally wound polyethylene pipes to be installed in shallow pits, usually backfilled with the excavated material They provide a relatively new alternative for conventional BHEs in cases of low heating demand and where normal (deeper) BHEs cannot be licensed They can also be applied to compensate for overly short-dimensioned BHEs Figure 11 shows an example and a system sketch

All systems need an electrical HP by which the low BHE output temperature (rarely above 10 °C) can be raised to the required level (35–50 °C, depending upon the heating system like underfloor panels) The smaller the increase of temperature needed, the higher the HP performance efficiency

7.06.3.3 The Core Piece: The HP

The ground provides an immense reservoir of heat, inexhaustible on human timescales Although the temperature level in shallow geothermal resources is only 10–20 °C, this level can be raised by an HP: this device converts the low-temperature heat of the ground to heat at higher temperature that can then be used for space heating, warming domestic water, and so on

Figure 12 shows the principal HP components and processes: (3) evaporator: heat uptake from the ground or ground­ water by a working fluid that evaporates; (1) compressor: compression of this gas, thereby increasing its temperature; (2) condenser: heat transfer to the heating circuit by condensation of the compressed medium; (4) expansion valve: expansion of the condensed working fluid to lower pressure The four components are connected to a closed circuit The working fluid usually is an organic compound with a low boiling temperature (e.g., tetrafluoroethane (R134a), –26 °C) In most cases, the compressor is driven by electric power As previously mentioned, the ratio heat delivered/electricity consumed is the COP The smaller the temperature difference between heat uptake and delivery, the higher the COP (cf Figure 18)

2

4

3

Figure 12 Heat pump components 1: compressor; 2: condenser; 3: evaporator; 4: expansion valve

7.06.4 Heating and Cooling with GHPs

As mentioned above, GHP systems can provide space cooling also In moderate climates, in summer, the ground below about 15 m depth is significantly colder than outside air Thus, a large geothermal store with favorable heat capacity is available where the heat can

be exchanged (extracted from the building and deposited in summer, extracted from the ground store and supplied to the building in winter) The thermal capacity of the system depends – excluding the volume – on the thermal and hydrogeologic characteristics of the installation site; these must be carefully considered in system dimensioning In summer, most of the time, the HP can be bypassed and the heat carrier fluid circulated through the ground by the BHEs and through the heating/cooling distribution (e.g., floor panels) By these means, the heat is collected from the building and deposited in the ground for extraction in the next winter (‘free cooling’) When free cooling alone cannot satisfy the cooling needs, HPs can be reversed for cooling since they can operate in normal (heating) and reverse (cooling) mode Both operations need electricity for the compressor Figures 13 and 14 show the normal

Heat exchanger refrigerant/air (Condenser)

Cool return air from conditioned Space

Expansion valve Refrigerant

reversing valve

Warm supply air to conditioned space

Heat exchanger refrigerant/water (evaporator)

Domestic hot water exchanger (desuperheater)

Domestic water

In

Out

(geothermal)

Figure 13 Heat pump in a geothermal heat pump, heating mode Source: Oklahoma State University

Heat exchanger refrigerant/air (evaporator)

Warm return air from conditioned space

Expansion valve Refrigerant

reversing valve

Cool supply air to conditioned space

Heat exchanger refrigerant/water (condenser)

Domestic hot water exchanger (desuperheater)

Domestic water

In

Out

Refrigerant compressor

To/from ground heat exchanger (Geothermal)

Figure 14 Heat pump in a geothermal heat pump, cooling mode Source: Oklahoma State University

HE

Bypassing the HP for free cooling

HP

BHE

+

−

Figure 15 Scheme of free cooling with a geothermal heat pump BHE, borehole heat exchanger; HE, heat exchanger; HP, heat pump User: the buildings heat/cold supply (hydronic or fan coil system)

and reverse modes of HPs, and Figure 15 the scheme of free cooling Figure 15 also shows the three main components of GHPs: (1) the heat source (in this case a BHE); (2) the HP; (3) the building’s heating/cooling system Small pumps, circulating

not shown

performance is the ground thermal conductivity Reliable values are needed for the design of large-scale systems These can be determined in situ Site investigations, by specific equipment and procedures (wireless temperature logger, repeated measurements, numerical model simulations), provide the vertical thermal conductivity profile along with the temperature profile These are especially needed for systems intended for space heating and cooling

The key factor dominating the performance is the heat exchange between BHE and the surrounding ground; it depends directly upon the ground thermal conductivity λ at the site in question λ is thus a key parameter in designing BHE-coupled GHP systems; the specific heat extraction rate (W per meter BHE length) is directly proportional to λ (see Table 1) This must be considered especially

in the design of BHE groups, that is, optimization of the BHE group by determining the BHE number, spacing, and depth Ground thermal conductivity λ must be determined in situ at the BHE/HP system site; sizing of the system needs to be implemented immediately after receiving the λ information This is usually performed in a special test BHE installed at the beginning of drill site preparation Laboratory determination of λ on rock samples from BHE drill holes is also possible, for example, on cuttings (see Reference 8), but it is time consuming

7.06.5.1 Conventional Thermal Response Test

Thermal response test (TRT) is the customary method to determine ground thermal conductivity in situ A standard TRT circulates a heated fluid in a test BHE and yields average values of thermal conductivity, thermal borehole resistance, and ground temperature over the BHE, by using a linear heat source model (for details see, e.g., Reference 9)

In the TRT, a defined heat load is put into the test BHE and the resulting changes of the circulated fluid are measured (see, e.g., Reference 10) Figure 16 shows the TRT scheme and Table 2 presents thermal borehole resistance values; the resistance rb governs

Table 1 BHE performance (single BHE, depth ∼ 150 m) in different rock types Rock type

Thermal conductivity (W m−1 K−1)

Specific extraction rate (W per m)

Energy yield (kWh m−1 yr −1) Hard rock

Unconsolidated rock, saturated Unconsolidated rock, dry

3.0 2.0 1.5

Max 70

45–50 Max 25

100–120

90

50

European Summer School on Geothermal Energy Applications Oradea, Romania (CD-ROM) [7]

acquisition Electric power T2

T1

Mobile test equipment

Figure 16 Schematic of a thermal response test T1, fluid input temperature; T2, fluid output temperature Reproduced from Lund J, Sanner B, Rybach

L, et al (2003) Ground source heat pumps – A world review Renewable Energy World July–August: 218–227 [5]

Table 2 Borehole thermal resistance with different grouting materials and polyethylene U-pipes

Type of BHE ( W m−1K−1) ( K m W−1)

Reproduced from Lund J, Sanner B, Rybach L, et al (2003) Ground source heat pumps

A world review Renewable Energy World July

the temperature losses between the undisturbed ground and the fluid inside the BHE pipes The plastic pipes in the BHE are cemented to the surrounding ground by a special grouting material; the higher its thermal conductivity λgrout, the lower rb

will be

For efficient design of large BHE arrays, more specific input data are needed (especially about the vertical variation of ground thermal properties) than average values New, innovative approaches enable determination of the vertical profile of ground thermal conductivity λ at a give site

7.06.5.2 Determination via Local Heat Flow Value

This method requires a high-resolution measurement of the vertical temperature profile For this purpose, the small and

the bottom of a BHE U-tube and records pressure (=depth) and temperature at preselected time intervals during descent After completion of the logging, the probe is flushed back to the surface by a small pump where the probe is connected to a laptop computer for data retrieval The measurement run for a 300 m deep BHE takes less than 60 min The instrument has a temperature resolution of
0.003 °C Further details like construction, calibration, field deployment, and data evaluation are given in Reference 11

In data processing, the λ profile of the logged BHE is calculated, with a regional heat flow value at hand, from the temperature gradient along the BHE to be derived from the measured temperature log From the measured temperature profile, the local geothermal gradient is then calculated layerwise (first derivative; (O Ti is the temperature gradient of depth section i)

Tu − T1

u − z1

where Tu is the temperature measured at the top (z = zu) and Tl at the bottom (z = zl) of interval i Finally, with the local terrestrial heat flow value qloc (obtainable from regional heat flow maps; in Switzerland, e.g., from Reference 12), the thermal conductivity of each individual depth section can be calculated:

qloc

i

Figure 17 shows an example of λ determination based on a local heat flow value On the left side, the temperature profile is displayed (black line) along with the profile of the temperature gradient The latter is given by a green line (original data with a constant Δz of 1.1 m) and by a light brown line (smoothed; sliding average over Δz = 13 m) The right side of Figure 17 displays the thermal conductivity profile as calculated by eqn [2] For comparison, the results of laboratory measurements of thermal conductivity are also given (black vertical bars) The agreement is remarkably good; thus the method of calculating the thermal conductivity profile from the temperature profile measured by the wireless probe yields highly reliable, in situ thermal conductivities

7.06.5.3 Enhanced Thermal Response Test

A conventional TRT yields only average values of the local ground thermal conductivity λ, over the BHE length For a more reliable dimensioning especially of large multiple BHE arrays, the vertical variation of λ is needed Besides, if a BHE system shall provide

ground but also the temperature–depth profile The enhanced thermal response test (e-TRT) is designed to yield both ground characteristics: λ(z) and T(z)

The new concept is based on repeated temperature measurement over the entire length of the BHE using the wireless temperature probe NIMO-T® mentioned above The temperature in the BHE is measured (1) before the response test and (2) after approximately