Choosing the Right Solar Heating- Freeze Protection in Closed Loop


In the previous article we have discussed about the solar heating close loop system basics and heat exchanger. In close loop system heat exchanger is the main component that is used to transfer the solar heat energy from solar collector to our home’s hot water system.

To give a clearer picture please check the diagram below, how the heat exchanger works within the close loop.

Previous articles:

  1. Choosing the Right Solar Heating- Passive, Active & Open Loop (Part 1)
  2. Choosing the Right Solar Heating- ICS & Close Loop Basics (Part 2)
Close loop system

Close loop system

Antifreeze in Solar Heating System

Air or antifreeze is the medium that is circulated within the loop. Normally ethanol, methanol,   propylene glycol and ethylene glycol is used as the medium. Antifreeze is used as freeze protection, to avoid water become ice which is occurred in open loop systems.  In the previous article, we didn’t discuss deeply about this issues.

Although some people might think that this information is only important to engineers or technician, however knowing about chemical properties of the media give you peace of mind.

It is very important to have a basic understanding so that when you go out for solar system shopping, you know what you want to buy. In addition, this is also good information for readers who would like to make their own system or a DIY system in the future.

Ethanol

Methanol

low corrosivenesshigh flammability if purelow toxicity

high heat transfer capacity

low viscosity

moderate cost

low corrosivenesshigh flammability if purehigh toxicity

high heat transfer capacity

low viscosity

low cost

Propylene Glycol

Ethylene Glycol

low corrosivenesslow flammabilitylow toxicity

high heat transfer capacity

higher viscosity

high cost

low corrosivenesslow flammabilityhigh toxicity

lower heat transfer capacity

higher viscosity

high cost

Viscosity

Lower viscosity is better so that the movement of the liquid medium is faster and more efficient to distribute heat energy from solar collector to our home.

Heat Transfer Capacity

Higher heat transfer capacity definitely is the best because more heat energy could be transferred to our home.

Water has the highest heat transfer as well as the highest viscosity comparing any of the antifreezes. However, during winter especially at night water could not be used as it would turn to ice. Antifreeze which is mixed with water is used instead.

Flash Point & Boiling Point

Flash point and boiling point are also important to know since antifreeze has maximum limit of temperature could work. If water has boiling point of 100 degree Celsius then antifreezes have different maximum limits.

Flash point is the lowest temperature where it will evaporate enough fluid to form a combustible concentration of gas. The flash point is an indication of limit that a chemical can burn.

Boiling point is the maximum temperature for the chemical before it evaporates.

Antifreezes Chemical Properties

Ethanol

Freezing Point of Ethanol + Water Solutions

Freezing Point

Ethanol Concentration(% by volume)

0

10

20

30

40

50

60

70

80

90

100

Temperature (oF)

32

25

15

5

-10

-25

-35

-55

-75

-110

-175

(oC)

0

-4

-9

-15

-23

-32

-37

-48

-59

-73

-115

Flash Points of Ethanol + Water Solutions

Flash Point

Ethanol Concentration(% by volume)

5

10

20

30

40

50

60

70

80

90

96

Temperature (oF)

144

120

97

84

79

75

72

70

68

63

63

(oC)

62

49

36

29

26

24

22

21

20

17

17

Warning: HIGH flammability if pure

Methanol

Freezing Point of Methanol + Water Solutions

Freezing Point

Methanol Concentration(%) by mass

0

10

20

30

40

50

60

70

80

90

100

by volume

0

13

24

35

46

56

66

75

83

92

100

Temperature oF

32

20

0

-15

-40

-65

-95

-215

-225

-230

-145

oC

0

-7

-18

-26

-40

-54

-71

<-73

<-73

<-73

<-73

Flash Points of Methanol + Water Solutions

Flash Point

Methanol Concentration(%) by mass

0

10

20

30

40

50

60

70

80

90

100

by volume

0

13

24

35

46

56

66

75

83

92

100

Temperature oF

-

130

110

95

85

75

70

60

54

54

54

oC

-

54

43

35

29

24

21

16

12

12

12

Warning: HIGH flammability if pure and HIGH toxicity

Propylene Glycol

Freezing Point of Propylene Glycol + Water Solutions

Freezing Point

Propylene Glycol Solution(%) by mass

0

10

20

30

40

50

60

by volume

0

10

19

29

40

50

60

Temperature oF

32

26

18

7

-8

-29

-55

oC

0

-3

-8

-14

-22

-34

-48

Because of slush creation, propylene glycol and water solutions should not be used close to the freezing points.

Boiling Points of Propylene Glycol + Water Solutions

Boiling Point

Propylene Glycol Solution(%) by mass

0

10

20

30

40

50

60

by volume

0

10

20

29

40

50

60

Temperature (oF)

212

212

213

216

219

222

225

  • T(oC) = 5/9[T(oF) - 32]

Ethylene Glycol

Freezing Point of Ethylene Glycol + Water Solutions

Freezing Point

Ethylene Glycol Solution

(% by volume)

0

10

20

30

40

50

60

Temperature (oF)

32

23

14

2

-13

-36

-70

(oC)

0

-3

-8

-16

-25

-37

-55

Because of slush creation, propylene glycol and water solutions should not be used close to the freezing points.

Boiling Points Ethylene Glycol + Water Solutions

Boiling Point

Ethylene Glycol Solution

(% by volume)

0

10

20

30

40

50

60

70

80

90

100

Temperature (oF)

212

214

216

220

220

225

232

245

260

288

386

(oC)

100

101.1

102.2

104.4

104.4

107.2

111.1

118

127

142

197

In the next article we shall read more on few types of solar collectors, how it works and some specifications of them. It shall be more interesting article because it is more mechanics than numbers like this article.

Although this article has so many numbers and may be boring, one day you might find it useful as reference when you are shopping for solar heating system. For readers who are interested to know about solar PV and want to get free quotes from the simply visit, FREE SOLAR QUOTES. Thanks for reading.

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