Post on 16-May-2022
1/48
Stagnation and components
planning
stagnation
fluids
pipes
insulation
2/48
Stagnation
state without a heat removal from collectors at incident solar radiation
causation:
storage temperature achieves the limit value, controller stops the circulation pump
blackout
incompetent intervention (closure of collector loop)
consequence
increase of temperature in solar collector
balance state: energy input = heat loss
collector achieves maximum temperature at given conditions
boiling of fluid, steam production
3/48
Stagnation temperature
depends on conditions
extreme (nominal) conditions: G = 1000 W/m2, te = 30 °C
calculation from intersection of efficiency characteristic with
horizontal axis
positive root of parabola
0
100030
G
ttt emstg
Ga
Gaaa
G
tt em
2
20211
0 2
4
4/48
Stagnation temperature
collector typetstg
[°C]
unglazed collector 50 - 65
glazed non-selective collector 90 – 110
glazed selective collector 150 – 180
vacuum tube collector 250 - 300
solar collector has to withstand it
temperatures could be lower dependent on real climate conditions
5/48
Stagnation behaviour
liquid is expanding
achieves boiling point (at given pressure), boiling starts
first bubbles appear, saturated steam, liquid is expelled from collector
rest of liquid is transformed in steam
volume of collector is filled by steam, high heat removal
superheating of steam in collector
emptying of collector, stable state, collector full of steam phase
decrease of radiation, decrease of temperature
condensation, liquid phase fills up the collector back
6/48
Stagnation behaviour
7/48
Emptying of solar collectors
good
bad
8/48
Back valve
wrongright
9/48
Heat transfer fluid
requirements
types
properties
10/48
Requirements
low freezing point, high boiling point
year-long operation (-25 to -30 °C)
stagnation (flat-plate 200 °C, vacuum tube 300 °C)
suitable thermophysical properties
high thermal capacity, high heat conductivity (heat transfer)
low viscosity (friction losses)
thermal stability, long-term stability
high temperatures (stagnation), phase change
elimination of aditives separation, decrease of pH
11/48
Requirements
protection to corrosion
anticorrosion additives (inhibitors), lifetime of system
compatibility with other materials
sealing, pipes
safety
nonflammable, nonexplosive, nontoxic, biodegradable
low price !
12/48
Types of solar liquids
water
nontoxic, nonflammable, cheap, high thermal capacity, low
viscosity
limited usable temperature range (seasonal systems),
ethylenglycol
antifreeze mixture with water, toxic, low viscosity
propylenglycol
antifreeze mixture with water, high viscosity dependent on
temperature, low thermal capacity (lower by 20 % than water),
corrosion inhibitors, stabilisers and other additives
13/48
Stagnation x propylenglycol
pH=8.2 pH=6.8
vacuum tube 300 °C
flat-plate 150 °C
usual solar liquids
regular testing of pH
14/48
Propylenglycol properties
2210
15,273 AAA
TF
2
54321
15,27315,27315,273
TA
TA
TAAAPx
2
54321
15,27315,27315,273ln
TA
TA
TAAAPx
freezing temperature TF
density, thermal conductivityspecific heat
dynamic viscosity, Prandtl number
source: Conde, M.: Thermophysical properties of brines – Models, Conde Engineering,
http://www.mrc-eng.com, Zurich 2002.
... mass fraction of glycol in
water mixture
15/48
Propylenglycol properties
Parameter
A
r[kg/m3]
cp[kJ/kg.K]
l[W/m.K]
m[Pa.s]
Pr
[-]
TF[K]
0 - - - - - 1,0
1 508,41109 4,47642 1,18886 -1,02798 6,66139 -0,03736
2 -182,40820 0,60863 -1,49110 -10,03298 -6,99440 -0,40050
3 965,76507 -0,71497 -0,69682 -19,93497 -18,55114 -
4 280,29104 -1,93855 1,13633 14,65802 12,04640 -
5 -472,22510 0,47873 0,06735 14,62050 14,47735 -
zdroj: Conde, M.: Thermophysical properties of brines – Models, Conde Engineering,
http://www.mrc-eng.com, Zurich 2002.
16/48
Propylenglycol properties
density specific heat
dynamic viscosity thermal expansion
17/48
Kinematic viscosity
0
1
2
3
4
5
6
7
20 40 60 80 100
t [°C]
[mm2/s]
propylenglykol+voda
voda
propyleglycol-water
water
18/48
Influence of solar fluid
on transferred heat power, heat exchangers
lower thermal capacity, laminar flow = lower heat output from HX
on circulation pump efficiency
hydraulic characteristic (based on water) will not change
decrease of pump efficiency due to higher friction losses in pump
size of expansion vessel
higher thermal expansion of propylenglycol mixture (50/50) than
for water
19/48
Pipes and insulation
requirements
materials
pipe sizing
insulation thickness
20/48
Pipes and insulation
pipes – interconnect source and load
insulation – prevent heat losses
21/48
General requirements
functionality
must respect type and use of the system
lifetime
resistance to pressures and temperatures, ageing, weathering
energy efficiency
pipes – low pressure loss, impact on electricity consumption of
pumps
thermal insulation – low heat loss, impact on efficiency of system,
need for back-up heating
22/48
Pipes - materials
plastic
only swimming pools
EPDM, polypropylen,
polyester, polyethylen
protection to UV radiation
23/48
Pipes - materials
copper pipes
low roughness (k < 0.00063)
laminar flow, not dependent on
roughness
(+) simple connection (soldering,
pressing)
(+) same material as collectors, zero
el.-chem. potential
(–) price, especially d > 28 mm
24/48
Pipes - materials
steel
higher roughness (k = 0,02 mm),
laminar flow
(–) welding, screws
(– – –) zinc pipe
chemical reactions zinc-glycol
(+) price, especially larger
dimensions
25/48
Pipes - materials
stainless steel (corrugated)
(–) higher pressure loss
(–) price
(+) hygienic atest for
potable water
(+) flexible (ro = 2D),
formable (kombiflex)
(+) quick and simple
installation
26/48
Pipes - materials
compact pipe systems
flexible or Cu pipes
mineral or rubber insulation
wiring for sensor
protection shield
27/48
Piping
recommendation
output from collector should be shorter (heat loss)
Tichelmann distribution x very low pressure loss of collectors
balance valves (thermal resistance !)
short lengths of pipes in external environment (need for resistant
insulation)
deaeration of liquid
28/48
Piping
29/48
Piping
thermal expansion - compensation
fixed points x dilatation device (U loop,
bends, compensators)
per 10 to 15 m
sliding point
compensator
fixed point
30/48
Pipe dimension
required flowrate in collector field
low-flow system: 10 to 20 l/h.m2
high DT at collector 25 až 40 K
high-flow system: 50 to 100 l/h.m2
low DT at collector 5 to 10 K
economic velocity, economic pressure drop
velocity in pipes 0.2 to 0.6 m/s (analogy to heating systems)
31/48
Pipe dimension
flowrate [l/h]
pre
ssu
re d
rop
[P
a/m
]
50
100
200
300
500
1000
1500
30
20
32/48
Pipe dimension
required size
Cu dimensions > 28 x 1.5 mm
very expensive
division of collector loop
required volume of loop
pipe volume as protection of expansion vessel against to steam
penetration at stagnation conditions
Vp = (0.5 to 1.0)Vk
0
500
1000
1500
2000
2500
12x1
28x1
28x1
,5
35x1
,5
42x1
,554
x264
x276
x289
x2
108x
2,5
Kč/m
33/48
Thermal insulation - requirements
resistance to ambient environment
humidity – increase of loss, degradation
closed cells
UV radiation – carbon aditives
birds – „tasty“ material
resistance to high temperatures
at collector: stagnation temperatures
min. 170 °C
distant places: min. 120 °C
34/48
Materials for thermal insulation
EPDM foams, syntetic rubber
(+) low thermal conductivity
(+) closed structure
(0) UV protection
(–) birds
resistance:
170 °C short-term
130 °C long-term
35/48
Insulated piping
36/48
Materials for thermal insulation
mineral wool (glass, stone)
(+) UV radiation
(–) open structure
need for sheathing (aluminium)
(+) long-term resistance to
280 °C
37/48
Thermal insulation sheathing
38/48
Pipe heat loss
izeee
ize
iz sdd
sdU
2
112ln
2
1
l
ew ttLUQ
[W/m.K]
typical values
insulation thickness siz = dimension de
39/48
0
20
40
60
80
100
120
6 9 13 20 25
tloušťka izolace [mm]
ná
kla
dy
[K
č/m
]
Insulation thickness
energy-economic optimisation
insulation price, energy price
economic thickness of insulation
operation
investment
total
thickness [mm]
annual costs
[C
ZK
/m]
40/48
Thermal bridges
clips
fittings, valves
pumps
heat exchangers
41/48
Example - continuation
solar system for the block of flats in Prague
42/48
Known inputs
20 pcs. of solar collector, A = 37.5 m2
volume of solar collectors 20 x 1.5 l = 30 l
collector loop
horizontal piping suterrain 15 m
vertical piping 20 m
horizontal piping roof 15 m
total one/way 50 m
total 100 m
43/48
Pipe sizing
low flow
10 – 20 l/h.m2
12 l/h.m2 x 37.5 m2 = 450 l/h
dimension
di = 16 mm (18x1 mm)
high flow
50 – 100 l/h.m2
70 l/h.m2 x 37.5 m2 = 2626 l/h
dimension
di = 39 mm (42x1.5 mm)
economic velocity 0.6 m/s
44/48
Pipe sizing
low flow
dimension 18x1 mm
volume 0.2 l/m x 50 m
total 10 l < 15 l (!)
high flow
dimension 42x1.5 mm
volume 1.2 l/m x 50 m
total 60 l > 15 l
prevention against steam penetration at stagnation
volume of one/way piping = 0.5 x collector volume (= 15 l)
need to increase of piping in case of low flow system
22x1 mm 0.314 l/m x 50 l 15.7 l > 15 l
45/48
Pipe heat loss
izeee
ize
iz sdd
sdU
2
112ln
2
1
l
ew ttLUQ
[W/m.K]
typical values
insulation thickness siz = dimension de
46/48
Insulation
low flow
dimension 22x1 mm
insulation thickness 25 mm
Rockwool PIPO 22 x 25 mm
high flow
dimension 42x1.5 mm
insulation thickness 40 mm
Rockwool PIPO 42 x 40 mm
selection from tables of mineral wool
47/48
Cost
low flow cost
pipe 22x1 175 CZK/m
ins. 25 mm 75 CZK/m
total 250 CZK/m
total 250 x 100 m = 25 kCZK
high flow cost
pipe 42x1.5 mm 491 CZK/m
ins. 40 mm 185 CZK/m
total 676 CZK/m
total 676 x 100 m = 68 kCZK
comparison of low-flow and high flow from the economic point
price difference by factor 3
48/48
Energy efficiency
heat loss coefficient
annual heat loss (simplified)
siz = 25 mm 40 mm
liz =1.1 x 0.036 W/mK (declared)
he = 15 W/m2K
U = 0.215 W/mK 0.226 W/mK
tm = 40 °C
te = 20 °C
LF: Q = U.L.(tm – te) * t = 440 W * 2000 h = 880 kWh/a
HF: 452 W .... 904 kWh/a