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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.
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