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