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