Energy saving effect by air circulation heat storage system

using natural energy

李 学成

1. Introduction

In the field of housing and buildings, the final

energy consumption is accounted for more than 30%

in Japan. The increase in energy consumption and CO2

emissions from the past is remarkable and plays a

major role in realizing a low-carbon society. Thus

further strengthening of energy conservation

measures is required1). In this study, the development

of a system is aimed at reducing sensible heat load

and peak load in a detached house by employing air

circulation type of central air conditioning system

using PCM UNIT, which has latent heat storage effect.

The system is maximized by applying the air

circulation through the roof ventilation layer, by

which heat exhaust and radiation cooling in summer

/ the heat collection in winter are performed. It

is expected that the system can be introduced into

ventilation layer provided in a conventional house.

2. Experiment on PCM BOX

A latent heat storage material (hereinafter

referred to as PCM: Phase Change Material) is a heat

storage material utilizing latent heat (heat

absorption during melting/heat release during

solidification) when a substance undergoes a phase

change from solid to liquid or from liquid to solid.

Figure 2.1.(a) shows the interior of the

experimental room, and Figure 2.1.(b) to (d) shows

the condition of PCM BOX. Small joists (thickness

6mm, width 25mm) are installed on both sides of PCM

packing component. The internal dimension of the box

is 300mm×300mm×300mm. The outside surface of the

XPS (Styrofoam) is affixed with an aluminum tape.

The number of ventilation can be regarded as 0

times/h because the box is sealed as much as

possible. The experiment is set to three levels: (a)

Blank (without PCM), (b) PCM 1st layer (PCM exposed

on the inner surface of the box), (c) PCM 2nd layer

(PCM placed on the back side of the floor).

(a) (b)

(c) (d)

Figure 2.1. Experimental conditions: (a) interior of

experimental room, (b) Case 1: Blank box (without PCM),

(c) Case 2: PCM_first layer, (d) Case 3: PCM_second

layer

2.2. Results of measurement and calculation

The experimental results were verified by using a

software THERB for HAM2), which is combined

simulation of heat and moisture transfer, and air

flow. The thermal environmental condition in the

experimental room is the temperature of 23℃ and the

relative humidity of 50% in curing time. When

measuring, the temperature is 23±7℃ (24 hour

period) and the relative humidity is 50%. Figure 2.2

shows the measurement result of the temperature. It

can be confirmed that PCM absorbs the heat during

melting in the heating process and releases the heat

in solidification in the cooling process through the

temperature change of the PCM surface. Thus, the

effect of thermal regulation and the peak

temperature reduction by PCM are found. The larger

storage capacity of Case 2, where PCM is exposed to

the air inside PCM BOX, is considered than Case 3,

comparing the temperature inside PCM BOX.

41-1

LDK

BathroomBathroom

Wash room

Wash room

Entrance

Location Yufuin, Oita Prefecture, Japan

Total floor area 235.61 m2

Area classification Area 5

Annual insolation area classification A3

Insolation area classification

in heating season H2

Mean heat transmission coefficient of

external wall 0.34/(m2 ∙K)

Japanese-style

room

Bedroom1

Bedroom2Bedroom3Bedroom4

Air conditioning

machine room

W.C.W.C.

Figure 3.1. Experimental house

Table 3.1. Overview of experimental house

Figure 3.2. Section of roof

Figure 3.3. Plan of experimental house

<Route ①>

Figure 3.4. Example of Summer route of air circulation

1F 2F

3. Analysis of effect of reduction in sensible heat load

3.1. Overview of experimental house

Figure 3.1 shows the exterior of the experimental

house and Table 3.1 shows the overview of the house,

which is located in Yufuin, Oita prefecture, Japan.

Figure 3.2 and Figure 3.3 show the cross section of

roof and the plan, respectively, and temperature

measurement point. Measurement of thermal

environments such as the internal temperature of the

roof ventilation layer (eaves, center, ridge), indoor

temperature and humidity, internal wall temperature

and humidity, external weather have been performed.

3.2. Air circulation route

(1) Summer mode

Figure 3.4 shows the air circulation route in the

summer mode. Firstly, since the temperature of the

roof ventilation layer can be expected to decrease

by radiative cooling at night, when the temperature

of the center of the roof ventilation layer is 25℃

or less, indoor air is taken into the roof

ventilation layer and blown to the PCM UNIT,

simultaneously with the cold storage in the PCM, and

5

10

15

20

25

30

35

40

1 2 3 4

Tem

per

atu

re [℃

]

1 2 3 4

Time [day]

1 2 3 4

1 2 3 4

Tem

per

atu

re [℃

]

Time [day]

Inside of box (measured) Inside of box (calculated)

PCM surface (measured) PCM surface (calculated)

Case 2:Back surface of plywood, Case 3 : Surface of plywood (measured) Case 2:Back surface of plywood, Case 3 : Surface of plywood (calculated)

Experimental room

Figure 2.2. Results of measurement and calculation. (left: Case1, center: Case2, right: Case 3)

41-2

0

5

10

15

20

25

30

35

40

0

1

2

3

4

5

6

7

8

9

10

19 20 21 22 23 24 25 26 27

Tem

per

atu

re [℃

]

Hea

t q

uan

tity

[kW

h/d

ay]

Time [day]

Cold storage of PCM BOX Cold release of PCM BOX

Average outside air temperature

0

200

400

600

800

1000

1200

1400

0

10

20

30

40

50

60

70

80

Time [day]

Sola

r ra

dia

tio

n[W

/m2]

Tem

per

atu

re[℃

]

Solar radiation Outside temperature

Central temperature of venilation layer Bedroom 2 temperature

10

20

30

40

50

60

70

-1600

-1200

-800

-400

0

400

800

Hea

t q

uan

tity

［W］

(Co

ld r

elea

se)

(Co

ld s

tora

ge)

Tem

per

atu

re[℃

]

Time[day]

Heat quantity in PCM BOX Outside air temperature

Center temperature in ventilation layer Bedroom 2 temperature

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

18 19 20 21 22 23 24 25 26 27 28 29 30 31

Tem

per

atu

re[℃

]Ti

me[

h]

Load

[kW

h/d

ay]

Time[day]

Cold storage amount [kWh/day] Sensible heat load of air conditioner ［kWh/day］

Latent heat load of air conditioner ［kWh/day］ Average temperature of outside air [℃]

Average temperature in the center of ventilation layer ［℃］ The time of temperature below 25℃ in ventilation layer [h]

Figure 3.5. Amount of radiative cooling

(August, 2016)

Figure 3.6. Change in heat quantity

(August, 2016)

Figure 3.7. Integrated quantity of

cold storage and release (August, 2016)

Figure 3.8. Temperature change in

roof ventilation layer (August 2016)

then the air is moved to the air conditioning room

(night cooling: route ①). Meanwhile, in the daytime,

the center temperature of the roof ventilation layer

becomes 25℃ or more, so indoor air is blown to PCM

UNIT by Fan B, without passing through the

ventilation layer, and then, the cold stored at night

is recovered and sent to the air conditioning room

(daytime cooling: route ②). At the same time, since

the roof ventilation layer becomes high temperature

due to solar radiation reception during the daytime,

the forced fan operates to take in outside air and

exhaust heat in the roof ventilation layer (daytime

heat exhaust: route ③). Through these air

circulation route, the effect of reducing the

sensible heat load in the daytime can be expected by

releasing the cold stored in the PCM at night

(2) Winter mode

Firstly, since the temperature of the roof

ventilation layer can be expected to increase by

solar radiation in the daytime, when the temperature

the center of the roof ventilation layer is 25℃ or

more, the indoor air is taken into the roof

ventilation layer by forced fan A, and blown to the

PCM UNIT, simultaneously with the heat storage in

the PCM. And then, the air is moved to the air

conditioning room (daytime heating: route ①).

Meanwhile, at nighttime, the temperature of the roof

ventilation layer becomes 25℃ or less, so indoor

air is blown to PCM UNIT by fan B without passing

through the ventilation layer, and the collected

heat stored at night is recovered and sent to the

air-conditioning room (night heating: route ②).

Through these air circulation route, the effect of

reducing the sensible heat load at nighttime can be

expected by releasing the heat stored in the PCM

during the daytime

4. Results of actual measurement and calculation

4.1 Analysis of actual measurement

Figure 3.5～3.8 show the effect in summer mode,

which is the radiative cooling, the change in heat

quantity in PCM BOX, the integrated quantity of cold

storage, and the temperature change in the roof

ventilation layer, respectively. The sensible heat

load is reduced in the summer because cooled air is

blown into the air conditioning room according to

the cold energy released a maximum of 6.6kWh/day by

PCM during the daytime. Figure 3.9～3.12 show the

effect in winter mode. The sensible heat load is

reduced in winter because heat air is blown into the

air conditioning room according to the hot energy

released a maximum of 6.2kWh/day by PCM during night.

41-3

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Tem

per

atu

re [℃

]

Hea

t q

uan

tity

[kW

h/d

ay]

Time [day]

Heat release of PCM BOX Heat storage of PCM BOX Average outside air temperature

0

10

20

30

40

50

60

70

-2,000

-1,500

-1,000

-500

0

500

1,000

Hea

t q

uan

tity

［W］

(Hea

t st

ora

ge)

(Hea

t re

leas

e)

Tem

per

atu

re [℃

]

Time [day]

Fan A Heat quantity in PCM BOX

Outside air temperature Center temperature in ventilation layer

Bedroom 2 temperature

0

5

10

15

20

25

30

0

5

10

15

20

25

30

10 11 12 13 14 15 16 17

Tem

per

atu

re[℃

]Ti

me

[h]

Load

[kW

h/d

ay]

Time [day]

Heat collection amount[kWh/day] Sensible heat load of air conditioner ［kWh/day］

Latent heat load of air conditioner ［kWh/day］ Average temperature of outside air [℃]

Average center temperature of ventilation layer ［℃］ The time of temperature above 25℃ in ventilation layer [h]

Solar radiation［kWh/m2・day］

0

100

200

300

400

500

600

700

800

900

1000

-10

0

10

20

30

40

50

Sola

r ra

dia

tio

n[W

/m2]

Tem

per

atu

re[℃

]

Time [day]

Fan A Solar radiation

Outside air temperature Center temperature of ventilation layer

bedroom 2 temperature Eaves side temperature of ventilation layer

Ridge side temperature of ventilation layer

Figure 3.11. Change in heat quantity

(November, 2016) Figure 3.12. Integrated quantity of heat

storage and release (November, 2016)

Figure 3.9. Temperature change in roof

ventilation layer (November, 2016)

Figure 3.10. Amount of heat collection

(November, 2016)

4.2. Comparison of measured and calculated value

Figure 3.13 shows the comparison of the measured

and calculated temperature change by THERB for HAM

in winter mode, which is susceptible spaces to air

circulation in the roof ventilation layer or the

amount of solar radiation. The calculated value

matches the measured value with high accuracy, and

the high accuracy of the calculation is confirmed.

4.3. Effect of reduction in sensible heat load

Figure 3.14 shows calculation result in order to

confirm the effect of the reduction in the sensible

heat load of the whole building in winter. Case 1

is each room air conditioning, Case 2 is air

circulation system using roof ventilation layer, and

Case 3 is case 2 with PCM UNIT. The rate of reduction

about 28% by using the system is evaluated in both

summer and winter in comparison with Case 1. The

total sensible heat load between Case 2 and Case 3

does not show clear distinction because PCM store

and release repeatedly in order to decrease peak-

load or overheating.

5. Conclusion

The effect of reduction in sensible heat of the

air circulation system with PCM UNIT is confirmed.

The heat storage amount of 6kWh/day and the

reduction in sensible heat about 28% was evaluated

in comparison of each room air conditioning.

Reference 1) METI, Ministry of Land, Infrastructure and Transport,

Ministry of the Environment : Interim summary on measures to promote "Housing and living for low-carbon society", July, 2012

2) Ozaki A. : Combined Simulation of Heat and Air and Moisture on the Hygrothermal Environment and Heating / Cooling Load，Technical Papers of Annual Meeting of IBPSA-Japan，pp.19-26，2005

0

1

2

3

4

5

6

7

8

Case 1 Case 2 Case 3

Hea

t lo

ad [

GJ/

per

iod

)

41-4

Figure 3.13. Comparison of indoor temperature change,

November, 2016 (From above, Bedroom2, Attic space, LDK)

Figure 3.14. Reduction in sensible heat load (January,

standard year, Extended AMeDAS weather data)

0

100

200

300

400

500

600

700

800

0

5

10

15

20

25

30

35

40

Sola

r ra

dit

ion

[W

/m2]

Tem

per

atu

re [℃

]Solar radiation Measured Calculated Outside temperature

0

100

200

300

400

500

600

700

800

0

5

10

15

20

25

30

35

40

Sola

r ra

dit

ion

[W

/m2]

Tem

per

atu

re [℃

]

0

100

200

300

400

500

600

700

800

0

5

10

15

20

25

30

35

40

3 4 5 6 7So

lar

rad

itio

n [

W/m

2]

Tem

per

atu

re [℃

]

Time [day]

0

0

0

0

0

0