design of closed loop borehole systems -(part 2€¦ · -show basic gshp hydraulic design...

Design of closed loop borehole systems - (Part 2)

Hydraulics

Robin Curtis – GeoScience / GSHPA

18th June 2020

Ground Source Heat Pump Association Webinar Series 2020

Objectives ?

- To raise awareness of the issue

- Illustrate the impact on performance

- Show basic GSHP hydraulic design approach

- Audience

(Closed loop) Ground source “hydraulics” - the issue

The forgotten bit….

Why does this matter ?

..protect the technology

CL GSHP Design

ffBuilding thermal loads

Ground parametersAvailable space Heat pumpFlow rate /max min EWT

Total borehole lengthSpacing

Code - for say 20To 50 years

Hydraulics

Costing

Drilling conditions

Contract terms

from GSHP Rogues Gallery >

Why ? - Do the numbers…

50kW heat pumpCOP ? ~4Electrical input = 50/4 = 12.5 kW

Pump power 11kW !!Effective COP = 50/(12.5+11) = 2.12 !!

(that’s the first bit of bad news)

The real bad news?

There’s nothing anyone can do about it.

Simple example -(of getting it wrong)

12 kW heat pump

Flow rate = 0.6 l/s

Told manufacturer = 3 x 60 metre boreholes in 32mm

Pressure drop per hole = ~15kPa (Re ~ 3200)

Pump selection = ~ Wilo 30/7

Simple example -(of getting it wrong)12 kW heat pump

Clever driller - decides on 1 hole.

180m of 40mm !!

Pressure drop now: ~120kPa (Re ~ 8000)

Pump size - off the graph > Parasitic power = huge!

Other warning signs

“We only use 40mm U-tubes…….”

“GSHP boreholes are always 100m deep……”

-10.00%

-9.00%

-8.00%

-7.00%

-6.00%

-5.00%

-4.00%

-3.00%

-2.00%

-1.00%

0.00%

3.00

3.05

3.10

3.15

3.20

3.25

3.30

3.35

3.40

3.45

3.50

1 2 3 4 5 6 7 8 9 10

Red

uctio

n in

SPF

%)

SPF

Pump power as % of compressor power

Effect of ground loop circulation pump on SPF

% reduction in SPF Reduced SPF

Where / how of ground loop heat transfer

Laminar flow in pipe

Ground

Ground

Pipe Wall

Pipe Wall

Boundary layer

Boundary layer

Ground Ground

GroundGround

= Heat flow

Non-Laminar flow in pipe

Ground

Ground

Pipe Wall

Pipe Wall

Boundary layer

Boundary layer

Ground Ground

GroundGround

= Heat flow

Reynold’s number

Re = ρVD / µ

ρ = densityV=fluid velocityD=hydraulic diameterµ =dynamic viscosity

(dimensionless - NO units !)

Laminar < 2000Turbulent > 4000

(illustrative only – do not use !

So > Increase Reynolds No:

Heat transfer improves

but……

Pump power also goes up

Designer’s hydraulic balancing act:

Maximise heat extraction/rejection

Minimise (circulation) pump power

7

Design Target

Ground loop circulating pump power< 2.5 - 3 %

of heat pump thermal output.(from MIS 3005)

(Prefer a few % of compressor electrical power : <5% )

Hydraulics -Pressure dropcomponents:

1) Active element(s)2) Headers3) Heat pump4) + contingency/fittings

Heat pump

Ground sideheat exchanger Php

Ground loopcirculating pump

Header pipeworkPh

Active elements(boreholes)Pa

Ground loop hydraulic components

Ground sideheat exchanger Php

Ground loopcirculating pump

Header pipeworkPh

Active elements(Slinkies)Pac

Heat pump

Ground sideheat exchanger Php

Ground loopcirculating pump

Header pipeworkPh

Active elements(horizontal loops)Pac

Hydraulics -“fixed” parameters

u minimum Heat Pump flow rate u minimum freeze protection limitu Pressure drop in heat pump – (beware !)

u Non-laminar – (transition zone) Re >2100

Antifreeze issues(another Webinar !)

uViscosity > low at low temperatures uConcentration for minimum freeze

protection

Hydraulics -the variables

u Borehole/loop lengths vs numberu Pipe diameter u Borehole pipe configuration (eg 1U / 2U)u Header arrangements

Heat pump thermal output H

Note B

Initial estimate of allowed pressure drop for complete ground loop circuit (kPa)

Circulating pump efficiency ?Normal~30%

High~50%

Permitted Pressure

Drop80kPa(PPD)

Permitted Pressure

Drop130kPa(PPD) Look up heat pump pressure

drop (kPa) = Php

Use appropriate "Active Element" graph to compute

pressure drop in one parallel active element (kPa) = Pa

Use appropriate hydraulic sizing graph(s) to compute pressure drop in header

pipes (kPa) = Ph

PD = 1.15*(Php+Ph+Pa)

is PD < PPD ?

Change no of active elements and lengths or change pipe diameter(s) orchange antifreeze

Go to pump manufacturer's pump selection.

Check pump electric input power < 2.5% of heat pump

thermal output H

Use higher

efficiency pump

Note A

Note C

Note D

Note E

Note F

Note G

Note H

Closed loop GSHP Hydraulic Sizing Flow Chart

Look up Heat pump Groundside flow rate F (litres/sec)

Select antifreeze

Note J

Note K

From ground loop sizing:Number of (parallel) active elements = N

Total Pipe length of EACH active element = L (metres)

Select appropriate Acive Element chart for type of pipe, antifreeze, and freeze protection level

Compute flow rate in each Active Element Fa = F / N (litres/sec)

On chart, locate flow rate Fa (horizontal axis)

Select required feeze protection level T (°C)

Does flow rate intersect desired pipe diameter ? (In turbulent zone)

Change pipe

diameter

Select proposed active element pipe outer diameter and pipe type

Change no of active

elements

Changeantifreeze

Read pressure drop (kPa) per metre of pipe on vertical axis = dP (kPa/m)

NO

Pressure drop for active element Pac = L x dP

NO

Flow chart to compute Active Element Pressure Drop

Note L

Note N

Note O

Note P

Note Q

Note R

Note S

Note T

Note M

Is Pac <( PPD - Ph)?

Go to calculateHeader Pressure Drop

Flow Rate (l/s)

Pres

sure

Dro

p (k

Pa/m

of P

ipe)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

Lam

inar

Flo

w Z

one

SDR11 Active Element - Ethylene Glycol (-10°C)

Look up Heat pump Groundside flow rate F (litres/sec)

Select antifreeze

Note A

Note K

Decide of no pairs of header pipes NHLength of header run LH (metres)

Select appropriate hydraulic chart for type of pipe, antifreeze, and freeze protection level

Compute flow rate in each pair of headers FH = F / NH (litres/sec)

On chart, locate flow rate FH (horizontal axis)

Select required feeze protection level T (C)

Does flow rate interesect desired pipe diameter ?

Change pipe

diameter

Select proposed header pipe outer diameter and pipe type

Change no of

headerpairs

Changeantifreeze

Read pressure drop (kPa) per metre of pipe on vertical axis = dP (kPa/m)

NO

Pressure drop for header pair Ph = 2 x L x dP

is Pac+Ph+Php < PPD ? NO

Flow chart to compute Header Pressure Drop

Note L

Note V

Note W

Note X

Note Q

Note R

Note Y

Note Z

Note U

Go to pump manufacturer's pump selection. Check pump electric input power < 2.5% of heat pump thermal output HNote H

Flow Rate (l/s)

Pres

sure

Dro

p (k

Pa/m

of P

ipe)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4

SDR11 Header Pipework - Ethylene Glycol (-10°C)

Pressure drop calculator

Input - pipe diameter (internal) flow ratepipe lengthviscosity

Output - Reynolds number,Turbulent pressure dropLaminar pressure drop

Simple example

Given: 20kW HP. HP flow rate = 0.95 l/s

Using EED or GLHEPRO or equivalent - get 1st attempt at Borehole depth, number and layout.

6 holes ~ 67m deep ie 400m of hole.

Simple example

6 holes ~ 67m deep ie 400m of hole.(flow rate = 0.16 l/s per hole)

Using PD calculator

3 common pipe sizes - 25mm, 32mm, 40mm:

Re ΔP25 = 2740 36 kPa 32 = 2140 11 kPa40 = 1710 4 kPa

Simple example

Re ΔP

25mm 2740 36 kPa32mm 2140 11 kPa40mm 1711 4 kPa

2nd go –

25mm – 7 holes 57m flow = 0.135 l/s Re = 2346 ΔP = 23 kPa32mm – 6 holes 67m flow = 0.16 l/s Re = 2140 ΔP = 11 kPa40mm – 5 holes 80m flow = 0.24 l/s Re = 2053 ΔP = 6 kPa

Repeat thermal calculations – iterate

Simple example

Component ΔP

Boreholes = ~12kPaHeaders = ~ 25kPa (depends on distance to plant room, and pipe diameter) Total = ~27 kPaAllow say 15% ~30kPa - fittings / manifolds / plant room etc.

(ok < 50kPa)

Add heat pump 10 kPa

For pump - need 40kPa (~ 4m head) at 0.95 l/s

Simple example

Pump selection parameters

Flow rate = 0.95 l/s

Working pressure / head = ~ 40 kPa

System curve + pump curve

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12Flow rate

Pre

ssu

re d

rop

or

Head

System curvePump curve

Pump sizing

Actual Graphic

Don’t blow it on the headers !!

eg 200m or 300 metre long runs. = Large, costly, pipe installs.

Does the plant room really have to be this far away ?

Remember - we are installing for 50 - 100 years.

Pumps will be running 2000 to 4000 hours per annum - for 50 years +

Once the ground array is installed - it is IRREVERSIBLE !

COP > ENERGY / CARBON / RUNNING COST

Don’t blow it on the headers !

Don’t blow it on the Manifolds

uUse properly designed low pressure drop GSHP manifolds

uNot underfloor heating assemblies!!

Variable speed pumps

and

high efficiency pumps

Variable speed pumps

Beware new pumps withautomatic speed controlbased on reducing flow as pressureIncreases.

Opposite of what is required for ground loop!

Miracle Nano-Fluids ?

Avoid on ground loops –

(salesmen don’t get it)

MCS Hydraulics Design Guide materials

https://mcscertified.com/standards-tools-library/

https://mcscertified.com/wp-content/uploads/2019/08/GSHP-Hydraulics-Design-Guide-.pdf

https://mcscertified.com/wp-content/uploads/2019/08/MCS-Hydraulics-Design-Pressure-Drop-Charts-v1.0-1.pdf

no more of these

please…..

Questions…..

and thank youwww.gshp.org.uk

Robin Curtisinfo@gshp.org.uk