WATER AS PHASE CHANGE MATERIAL IN HEAT STORAGE MAGAZINES FOR houses

8/10/2019 WATER AS PHASE CHANGE MATERIAL IN HEAT STORAGE MAGAZINES FOR houses

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(1,16 Wh/kg/C). Heat content of any reservoir utilizing sensible heat can be calculated as

follows:

Hs = cp*t*m* (1)

wherem = mass of the storage volume, t= temperature difference between loaded and

empty heat storage magazine and is the efficiency of the energy storage system.Thus when the sensible heat only is used for energy storage, 1m water 90C warm can

theoretically release 104,7 kWh of useful heat after cooling it to 0C.

2.1.2. Latent heat storage.

Latent heat of materials (phase transition heat) is usually much higher than the specific heat.

Thus the condensation of water vapour releases 2520 kJ/kg (700 Wh/kg) at10C and freezing

of water releases 334 kJ/kg (93 Wh/kg). The difference between magazines utilizing sensible

heat only or sensible and latent heat is not only in a larger heat capacity of the latter, but also

in the time independence of the magazine capacity utilizing Phase Change Materials (PCM).

PCM as energy storage media are richly represented in the literature. European patentdatabase (Esp@cenet) contains more than 6000 citations of the term Phase Change Material

and Internet Database (Google) returns over 7 millions citations. Systematic treatment of

PCM for heat storage can be found in (Semadeni, 2003; Raoux 2008; Mehling 2008).

Water as PCM for heat storage magazines is used scarcely in spite of the large heat content

and unlimited access. The reason may be the volume changes during the phase transitions and

low temperature of the fusion heat. Water expands about 2, 2 % (linearly) when freezing and

expands about 1330 times at 10 C after evaporation at an atmospheric pressure. The huge

volume change makes it unpractical to use liquid to gas transition of water as heat storage

medium in closed systems (1 kg water vapour stored at 100 C in 10 l vessel requires a

container withstanding the pressure at least 200 bar), but this phase transition occurs daily in

the atmosphere. Water vapour content in the saturated air is about 0,4% at 0C, 0,8% at 10C

and 1,5% at 20 C (w/w) or 1 kg H2O in approx. 130 m of the humid air.

The average temperature measured in Stockholm area (year 2007; 5927N, 1745E) was

8,0C and the average relative humidity was 82,2%. Average retrievable energy in each 100

m air is thus at least 2800 kJ (786 Wh), if the heat retrieval system can be cooled to

temperature -5C. The sensible heat of the air contributes then with 56%, condensation of the

air moisture with 36% and the fusion heat of the condensed water with 8 % of the total

content of usable heat in the air. While the fusion heat of condensed water does not contribute

substantially to the heat retrieval from the moist air, the situation becomes very different,

when the heat is retrieved from the humidity in the soil.1 m soil, saturated to 50% with the

water 15C warm, can release more than 60 kWh when cooled down to -0C. A small pit,5x10x1 m, filled with humid soil, can thus supply enough heat for a single family house

during two to three winter months, if the water in the soil is allowed to freeze.

3. SYSTEM DESCRIPTION

A domestic heating system, covering energy needs for a family house during a whole year,

based on usage of solar energy, has to utilize both direct insolation (mainly for summer hot

water need and loading of the seasonal heat magazine), heat saved in humid air (during

nights, autumn and spring months) and heat stored in the ground magazine during the winter

months. Such a system has to contain a heat pump and open solar heat collector connected in

such a way, that the cold, expanded fluid in the heat pump circuit can cool the heat absorbing

surface of the SHC. The solar heat collector obtains thus double function: it collects theinsolation and in the absence of the solar radiation it collects the heat from the air.


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Figure1,legend: 1: SHC;2:three way magnetic valve; 3: flat heat exchanger;4: circulation

pump;5: earth loop, 60m Cu tube;6: heat pump compressor;7a, hot water magazine;7b:

floor heating pipe;8: throttle;9: soil heater;10: flow meter;11: back flush valve;12:

irrigation tubing.

When the heat pump does not work and the temperature difference between the top of the

SHC and the middle of the pit exceeds 3C, the brine circulates between 1, P ad B ports of thevalve 2, 7a, 9, 11, 10, 4 and 1.The soil in the pit is heated. When the temperature difference

falls to less than 2C, the pump 4 stops the brine circulation. When thermostats, controlling

the hot water or room temperature activate the heat pump, the valve 2 connects ports P to A

and the circulation pump 4 starts the brine flow. The liquid circulates between SHC 1, ports P,

and A of the valve 2, flat heat exchanger 3, flow meter 10, pump 4, back to SHC 1. The

circulation of the brine continues as long as the heat pump 6 works and as long as the

atmospheric temperature is higher than -5C. Both sides of the SHC are cooled below the dew

point of the air and the sensible heat of the humid air and condensation heat of the water

vapour is acquired. The expanded, cold heat transfer fluid in the heat pump circuit receives

the heat from the brine in the heat exchanger 3 and transports it to the loop 5 in the pit.Depending on the temperature difference between the fluid and the soil surrounding the loop,

the fluid either heats the soil, or is heated, maintaining the stable working conditions for the

heat pump.

The heat capacity of the pit is dependent on the moisture content of the soil. Therefore all

rainwater from the roof of the house is led to the drainage tube system 12. At the end of

November 2008, the water content of the soil in the pit was ~65% w/w.

3. RESULTS AND DISCUSSION

Energy consumption of the house, before the installation of described heating system, was

~6500 kWh/year. Thermal properties of this house after the installation of the solar heating

system are described by the equation 2 and are illustrated in figure two:

y = -0,6772x + 14,248; R = 0,9572 (2)

Sept 08 - Feb 09

y = -0,6772x + 14,248

R2 = 0,9572

6

8

10

12

14

16

18

-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

Temp C

kWh/day

Figure 2: Measured thermal Properties of a House at the Latitude 5927 N

The measured mean atmospheric temperature in the year 2008 at the house site was 8,01C.

The calculated energy consumption is therefore 3229 kWh/year, which means that the

achieved energy savings are about 50%.

Solar energy distribution and energy requirements in this house can be seen in the Table 1.


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Table 1: Energy Distribution during the Year (kWh).

Jan Feb March Apr May June

Energy Demand kWh -852 -755 -816 -500 -205 -227SUN kWh/5m 415 1170 2053 2681 3074 3195

Energy in the Moisture 139 117 98 150 216 319

Energy in the Air 620 634 602 1087 1582 1918

Sum Air+Sun kWh/5m 1173 1921 2753 3919 4872 5432

Month Jul Aug Sept Oct Nov Dec Sum

Energy Demand kWh -213 -280 -400 -535 -765 -949 6500

SUN kWh/5m 3265 2092 1678 1207 631 308 21770

Energy in the Moisture 446 507 332 281 160 123 2887

Energy in the Air 2161 1880 1398 1148 668 524 14224

Sum Air+Sun kWh/5m 5872 4480 3408 2637 1459 954 38882

The solar energy values in the table are calculated from measurements of solar irradiation, air

temperatures and humidity. Values are based on the SHC area 5 m and -5C cold surface.

The air volume passing the collector during each night is calculated to 500m. The electrical

current consumption (year 2007, before the installation of the system) was weekly read off

from the wattmeter of the house. If one assumes a 50% yield of the air energy collection in

the SHC , then the energy demand of the house can be retrieved from the air between March

and the end of October. The energy for heating of the house during winter months

(about 4000 kWh) has to be retrieved from a heat storage magazine.

Figure 3. Temperatures in the pit between December 2007 and March 2009.

The temperature of the moist soil in the pit, 1 m below the surface (figure three), was at the

beginning of November 7,2C and the water content of the soil was 65%. The calculated

nominal heat capacity of the pit was 67 kWh/m, inclusive the latent heat of the soil humidity.

The absorbed solar heat, transferred during the summer months to the soil was ca 15000 kWh.This heat amount was partially lost during cold Scandinavian nights, but a part of it was


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conserved in the pit and surrounding. The heat stored in the surrounding of the pit increased

obviously the nominal capacity of the pit. The total heat capacity of the soil heat reservoir was

therefore estimated to 3000-4000 kWh, which should be sufficient for four winter months.

The temperature in the pit sank at the end of January to -1,5C and the entire volume of the pit

was frozen. The thermal conductivity of the compact material increased and the temperature

of the pit started to be more dependent on the air temperature. So on February 21st droppedthe temperature in the pit to lowest value of the season, -4,3C simultaneously with the

measured lowest temperature of the air, -10,9C . But cold winter nights are usually preceded

or followed by sunny days, which contributes to fast temperature increase in the heat

magazine.

4. CONCLUSIONS

a. Cooling of the SHC surface with the cold heat transfer fluid in the heat pump circuit to

temperatures below 0 C makes it possible to collect the heat from the air even during

nights and cloudy, cold days.

b. Irrigating the soil in the pit with the rain water and dense placement of tubing collectingthe heat for the heat pump allows the utilization of fusion heat of water. This means, that

the capacity of the heat storage magazine substantially increases and the land area of the

pit can be correspondingly decreased. Our first achieved results indicate, that an energy pit

5x10x2 m, containing wet soil with 60% of water can store more than 60 kWh/ m of

utilizable energy. And the useful energy density can be >100 kWh/ m land area.

c. Using energy wells in rocks as heat source for the heat pumps is possible only in

landscapes, where the soil depth is only few meters. In addition to it, only specific heat of

the rock material can be utilized for energy storage. The low storage heat capacity requires

therefore very deep wells, which makes large capacity magazines very expensive. An

alternative method, burying the energy collection loop into the soil, requires so far large

ground areas, because the formation of permanent frost in the ground has to be avoided.Utilizing the large heat of fusion with active melting of the frozen water, as described in

this article, solves the problem with the permafrost and substantially diminishes necessity

of large land areas or deep rock wells for obtaining sufficient heat capacity. The solution

for seasonal heat storage, utilizing fusion heat of water with active melting of ice is

therefore applicable not only for row houses with small grass plots, but also for apartment

houses.

REFERENCES

Howard C. Hayden, The Solar Fraud, p.118 ff, Vales Lake Publishing 2001, ISBN 0-9714845-0-3.

Energy in Sweden. Facts and Figures 2008, Table 25; Swedish Energy Agency 2008http://www.naturvardsverket.se.

http://ep.espacenet.com

http://www.google.com

M.Semadeni, Energy Storage as an Essential Part of Sustainable Energy Systems. Working Paper No 24,May 2003; CEPE ETH Zentrum WEC Zurich; (www.cepe.ethz.ch)

Simone Raoux and Matthias Wuttig Editors, Phase Change Materials: Science and Applications, Springer

2008. ISBN-13: 978-0387848730.

Harald Mehling and Luisa F. Cabeza, Heat and Cold Storage with PCM, Springer 2008,

ISBN: 978-3-540-68556-2

www.texsun.sewww.megatherm.se

EMS Brno, Czech Republic; model VV.www.emsbrno.cz

www.kippzonen.comEMS33 from EMS Brno, Czech Republic; www.emsbrno.cz.
http://ep.espacenet.com/http://www.cepe.ethz.ch/http://www.texsun.se/http://www.megatherm.se/http://www.emsbrno.cz/http://www.kippzonen.com/http://www.emsbrno.cz/http://www.emsbrno.cz/http://www.kippzonen.com/http://www.emsbrno.cz/http://www.megatherm.se/http://www.texsun.se/http://www.cepe.ethz.ch/http://ep.espacenet.com/

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WATER AS PHASE CHANGE MATERIAL IN HEAT STORAGE MAGAZINES FOR houses