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IDES-EDU modul

Lecture #5

Energy production

Coordinator: Sašo Medved, UL

Contributor: Sašo Medved, UL

Ground heat exchangers for air pre-heating and pre-cooling

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Note: Some of the pictures in the presentation are used from other sources which are citated. Some pictures are download from web and authors are unknown. We would like to thank all known and unknown authors. Some schemes are taken from marketing and technical material of different companies to improve the quality of presentation and we want to make acknowledgment to those companies. This presentation should be used for education purposes only.

Intro

Ground heat exchangers are one of the “low-ex” technologies used for heating and/or cooling of the buildings.

„LowEx“ systems can be defined as systems, which enabled the use of low-quality energy as energy source; this mean low temperature source for heating and high temperature source for cooling. Heat contained in surrounding air or in the ground is example of such energy source.

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Indoor comfort in the buildings can be achieved with low exergy energy sources like solar or

geothermal energy, environmental heat; this energy sources are more environmental friendly

and on the long run cheaper, than high exergy sources like fossil fuels or electricity.

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Intro

Ground heat exchanger (GHX) are made from tubes or channels buried horizontally 1 to 2 m below the surface. Ground heat exchangers can be dig in vertically into the ground, in this case depth between 50 to 100 m are common.

Polypropylene tubes for small and concrete tubes for large systems, as well as concrete channels are usually used.

Heat transfer fluid is most cases ambient air, but can be water as well.

Ground heat exchangers can operate as open looped system, in this case air is used as heat transfer fluid, or as close loop system. In this case water is common heat transfer fluid.

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Ground as energy source

Regarding to the temperature, ground can be divided in three layers: shallow sub-surface layer (up to 20 m below the surface), mid layer (20 to 50 m) and deep layer (bellow 100 m).

In sub-surface layer strong influence of solar radiation and ambient temperature is noticed; that’s why temperature in ground up to 20 m deep varies periodically with period of one year.

In mid layer, temperatures are constant (no gradient is present). Below this layer, temperatures are not time dependant and slightly rise with deepness. Gradient dT/dz depends on thermal properties of ground, under ground water presents and geological structure. Average values are between 30°C/km and 80°C/km.

grou

nd d

eept

h

20 m

0 m

50 m

-20°C 20°C

groundtemperature

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Ground as energy source

In most cases, ground heat exchangers are installed in sub-surface layer. Yearly temperature variation of this layer can be calculated regarding to:

average yearly ambient air temperature Te,av (°C)amplitude of ambient air temperature Ae (°C) number of days after of 1st of January when minimal ambient air temperature appears (∆n).

climate DD20(Kday/a)

Te,av (°C)

Ae (°C)

∆n (day)

Mediterranean 1880 13,8 9,0 372700 10,8 9,5 37

Continental 2980 9,7 10,5 353200 9,2 11,0 35

Alpine 3500 8,2 11,0 32

Typical value of influence meteorological parameters for different climates

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Ground as energy source

Beside meteorological conditions, thermal properties of teh soil have gread influence on the unsteady ground temperatrues. Thermal properties (density ρsoil, conductivity λsoil and specific heat capacity cp,soilcan be combined into soil thermal diffusion asoil.

Type of soil λsoil(W/mK)

asoil(m2/s)

Dry river sand,desert sand

0,27 2,22 10-7

Clay 1,30 1,01 10-6

Granit 2,79 1,37 10-6

Silicious sand 5,38 1,84 10-6

High thermal conductivity is reason why surroundings of the ground heat exchanger is filled by silicious sand

When meteorological conditions and soil properties are known, average daily ground temperature T(Z,n) at point Z (m) below surface for specific day n in the year can be determinated:

[ ]= − ⋅ °K(Z,n) e,av eT T A e C

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Ground as energy source

Exponent K is equal to:

where a*soil represents reduced thermal diffusivity (conversion of the unit from m2 per second to m2 per day:

Daily variation of T(Z,n) at Z=0m, 2m and 4m for teh site with Te,av 9,7°C and Ae 10,5 °C

( ) [ ] π ⋅π = − ⋅ ⋅ − ∆ − ⋅ π ⋅

1/2

* *soil soil2 Z 365K Z cos n n 1365 2365 a a

= ⋅ ⋅ ⋅ 2

*soil soil

ma a 60 60 24 day

600 120 180 240 300 360

T(Z,n)=T(0,n) = Te,n (Z=0m)

10

5

15

20

25

tem

pera

ture

T (

°C)

(Z,n

)

day

0

T(Z,n)=T(2,n) Z=2m

T(Z,n)=T(4,n) Z=4m

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Technical solutions

Open loop air ground heat exchanger. Single tube ground heat

exchanger for pre-heating of ventilation of for single family

building (max. air flow rate 250 m3/h) (left).

Intake of fresh air (bottom)

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Technical solutions

Technical characteristics and operation principles of multy-paralel tube open cycle air ground heat exchanger. If such system is used for pre-cooling, bypass for night-time summer cooling must be installed as it is shown on the picture.

Technical solutions

Close loop water ground heat exchanger. In this case pump, expansion vessel and heat exchanger in ventilation system is needed. Main advantage of such systems is much lower electricity consumption for running pump in comparison to ventilator in open loop systems.

Two parallel polietilen tubes are installed for enlarging of heat transfer surface

Additional pump and expansion vessel are needed (right) as well as additional heat

exchanger which is installed in ventilation system (most right)

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Advance technical solutions

In advance systems, phase change materials could be used. In this way temperature of supply air is more constant during the day. Supply air could flow thought pipes inside concrete core and thermally activate such construction.

PCM storage could be connected with ground heat exchanger (most right figure shows packages of encapsulated PCM. In this way sufficient heat transfer area is provided.

Distribution pipes could be inside concrete floor. In this way concrete construction is thermally activated, providing constant heating/cooling effect.Source: xia inteligente arhitecture, 2009

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Modelling of GHX - steady state operation

Outlet air temperature Tair,out can be calculated regarding to air inlet temperature Tair,in (= ambient temperature Tam), air mass flow rate mair, length L and diameter (D) of tube and soil temperature Tsoil.

The heat flux transferred by air flowing through a buried pipe can be written as:

( ) [ ]= ⋅ ⋅ −& &air air p air,out air,inQ m c T T W

Heat flux transferred by air is equal to convection heat flux from the surface of the pipe:

[ ]= ⋅π ⋅ ⋅ ⋅∆&123

pipe

air lnA

Q h D L T W mair air mass flow rate (kg/s)cp specific heat capacity at

constant pressure (J/kgK)Tair,in air temperature at inlet of GHX (°C) Tair,out air temperature at outlet of GHX (°C) ∆Tln logarithmiGHX temperature (°C) h convective heat transfer coefficient at

inner surface of GHX pipe (W/m2K)D pipe diameterL length of pipe

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Modelling of GHX - steady state operation

If we assume that Twall = Tsoil, effectiveness of air type GHX can be written as:

( ) [ ]⋅ ⋅ − = + − ⋅

64748

&

NTUh A cpmairair,out wall air,in wallT T T T e W

Solving equation for Tair,out results to:

( ) [ ]−−

ε = = −−

air,out air,in NTUsoil air,in

T T1 e 1

T TNTU number heat transfer units

Number of heat transfer units can be normalized per 1 m length of GHX (NTU/L). Cucumo at all published in IJHMT (2008) chart that can be use for determination of NTU/L for air GHX if Tsoil, D and volume flow rate V (m3/h) are known:

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Modelling of GHX - steady state operation

NTU/L can be read from chart as function of pipe diameter (650 mm) and volume flow rate (1200 m3/h) (NTU/L -> 0,019)

( ) ( )( ) ( ) [ ]− −−

= − → = + − ⋅ − °−

air,out air,in NTU NTUair,out air,in soil air,in

soil air,in

T T1 e T T 1 e T T C

T T

NTU/L

For L = 140 m, NTU is equal to 1,33

For Tsoil 12°C and Tair,in 2°C, Tair,out is equal to 9,3°C

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Transient operation of GHX - data from field measurements

This is how GHX shown on the pictures operates in real conditions, PE pipe U-shape, D 110 mm, L 80 m, V 80 m3/h. Obvious heat transfer in GHX is transient.

Summer week

Winter weekAutom week

Temp

eratur

e (°C)

Temp

eratur

e (°C)

Temp

eratur

e (°C)

Ambient

Tair,out

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Modelling of GHX - transient operation

Several custom made computer codes are available. For complex modelling, such as integration into the buildings thermal response modelling and integration into the building service systems, TRNSYS energy simulation software package is one of the most advance tool.Conjunction with Soil Model for Buried Horizontal Pipe (TYPE 711) and Pipe/Duct Model (TYPE 31)Soil model is defined with three dimensional finite element grid of soil around the pipe and temperatures are solved the system of resulting equations for the temperature of each element

Nodding scheme of the a three dimensional finite element grid surrounding the pipe (axial and end view) TRNSYS 16, a TRaNsient SYstem Simulation program, Solar Energy Laboratory, University of Wisconsin-Madison, 2005

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Modelling of GHX - transient operation

Required parameters: diameter and length of pipe, buried pipe depth, thermal performance of soil, number of radial, axial, circumferential of nodes. User have to enter the average surface temperature (usually the average annual air temperature), the amplitude of the surface temperature (the difference between the maximum annual surface temperature and the average annual surface temperature), and the day of the year on which the minimum surface temperature occurs. Required inputs are pipe U-value, fluid temperature and mass flow as well.Parameters, Inputs and Outputs of Soil Model for Buried Horizontal Pipe (TYPE 711)

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Modelling of GHX - transient operation

Parameters, Inputs and Outputs of Pipe/Duct Model (TYPE 31)

List of outputs: temperature of soil for each node, average temperature, pipe heat transfer, outlet air temperature, outlet flow rate

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Ground heat exchangers - examples

Concrete tube ground heat exchanger (U-tube type) with diameter of 800 mm and length 75 m for pre-heating of ventilation air in Shopping centre. Air flow rate 2800 m3/h.

Tubes are mounted under the small angle. This enables cleaning of inner surface and runaway of the (potential) water. Here difference in depth of channel can be notes at the beginning (left) and at the U-turn (right).

This is how the concrete tubes look likes. Two channel are connected in U-tube form.

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Ground heat exchangers - examples

Ambient and soil temperature on the

site

-15-10

-505

101520253035

1 731 1461 2191 2921 3651 4381 5111 5841 6571 7301 8031 8761hours [h]

Temp

eratu

re [°

C]

ambientdeepness 1,7 mdeepness 2,3 mdeepness 2,9 m

Outlet and inlet air temperature difference

-10,00

-5,00

0,00

5,00

10,00

15,00

1 731 1461 2191 2921 3651 4381 5111 5841 6571 7301 8031 8761hours [h]

Tair,

in- Ta

ir,ou

t (°C

)

deepness 2,9 mdeepness 2,3 mdeepness 1,7 m

Heat / coldness transfered into the buildingDeepness of GHX 1,7 m 2,3 m 2,9 m

Heat (kWh/a) 8286 9328 10239Coldness (kWh/a) (-)4551 (-)5311 (-)5943

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Ground heat exchangers - examples

Ground heat exchanger of Office building. Six parallel polypropylene tubes with length of 35 m and diameter of 315 mm are connected with tube 500 mm in diameter. Special pipes with antibacterial inner layer (AWADUKT) produced by REHAU company are used for sanitary reasons. Configuration was optimized regarding to heat transfer surface area, heat transfer coefficient (air flow rate) and pressure drop (electricity demand for running the ventilator).

Six parallel tubes are connected with connection pipe (here intake side can be seen) and buried 1,5 m bellow finished surface.

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Planning of vertical GHX– rule of tumb

In engineering practice vertical ground heat exchanger are planed regarding to specific heat flux that can be continuously extracted from ground. It depends on depth of GHX bellow the surface, thermal conductivity of soil and yearly operating time. Vertical GHX Specific heat flux

(W/m)Operating time 1800

hour/year

Specific heat flux (W/m)

Operating time 2400 hour/year

Dry sand < 25 < 20 Wet sand 65-85 55-65Clay 35 -50 30 -40Granite 65 – 85 55 – 65Sand with strong water current

80 - 100 80 – 100

Vertical GHX consist of two U tube installed in borehole with diameter of 100 mm. If ground are dry, hole is filled with siliceous sand. Values are valid for individual GHX in length of 50 – 100 m and in case distance between neighbour GHX are more then 6 m. Shorter period for geothermal heat recovery in case of longer operation period is reason for lower specific heat flux.

Drilling machine with

bound of PE tubes.

PE tubes in double U-

shape form installed into

the drilled hole

- examples

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During the summer time operation, ventilation air is cooled down. If temperature and humidity of ambient air if high, condensation of water vapour can occur. There is a role: relative humidity of ventilation air in GHX must be kept bellow 80% ! Nevertheless intensive microbiology research shown bellow conclude, that even after long (10 days) period of non-operation during the summer and autumn, colony forming units (CFU) of bacteria and mould did not exide values in ambient air !

Health issues

Bacteria growing environment Experiment Incubator Two colonies of bacteria (CFU=2)

Presence of microorganisms was determined at the exit of the air type GHX during continuous operation and immediately after 10 days break of operation. The microbiological culture particles (bacteria and mould) growing was analyzed.

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It was found out that number of CFU in the outflow air from GHX did not exceed the number in ambient air despite using pipe without antibacterial layer coating) of inner pipe surface ! We can conclude, that air type GHXs are not harmful, even if they operate in conditions with potential risk of condensation.

Health issues

Number of colony forming units (CFU) and CFU/m3

CFU CFU/m3

Continuous operation 10th AUG (V = 72 kg/m3)CFU in ambient air 3 – 5

Bacteria 3 – 4Mould 1 - 9

0,13 – 0,270,11 – 0,99

After 10 days break 20th AUGCFU in ambient air 3 – 5

Mould 1 – 6 0,11 – 0,66

After 10 days break 24th NOVCFU in ambient air 3 – 5

Mould 2 – 5 0,28 – 0,56

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In what form the GHX can be made ?How GHX works during the year ?How ground temperature during the year could be modelled ?Describe the meaning of NTU !Describe how all-year analyses of GHX operation can be performed ! What you know about safety issues of GHX operation ?

Self evaluationLOTZ

Literature/References

Incropera F., DeWitt D.; Fundamentals of Heat and Mass Transfer, Wiley, 1996B. Lenz, J. Schreiber, T. Stark; Sustainable building services, Detail Green Books; Germany, 2011xia inteligente arhitecture, 2009, Alexander Koch GmbH, GermanyM. Cucumo, S. Cucumo, L. Montoro, A. Vulcano; A one-dimensional transient analytical model for earth-to-air heat exchangers, taking into account condensation phenomena and thermal perturbation from the upper free surface as well as around the buried pipes,International Journal of Heat and Mass Transfer, Volume 51, Issues 3-4, February 2008, Pages 506-516TRNSYS 16, a TRaNsient SYstem Simulation program, Solar Energy Laboratory, University of Wisconsin-Madison, 2005Medved S.; Research report, UL, 2010

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