Download WATER AS PHASE CHANGE MATERIAL IN HEAT STORAGE

WATER AS PHASE CHANGE MATERIAL IN HEAT STORAGE MAGAZINES FOR
HOUSES.
J. Eyem
Instchemas AB,
Lädersättravägen 45
17670, Järfälla, Sweden
Tel: +46-70-5134501
[email protected].
ABSTRACT
A heating system for family houses, consisting of directly connected Solar Heat Collector
(SHC) to the heat pump was designed. The connection makes it possible to utilize both direct
solar irradiation and heat contained in the moist air during periods without solar radiation.
The designed system includes a seasonal heat storage magazine consisting of the pit filled
with a humid soil. The dense placement of the heat collector in the pit causes freezing of the
water in the soil, which contributes to high content of usable energy, more than 60 kWh/m³ of
the soil. The frozen soil is melted in the spring by means of warm brine from the SHC
flowing through the pipeline in the pit.
1.
INTRODUCTION
Solar radiation is the most abundant source of useful energy on the Earth. In spite of that, it is
not used in large extent for covering energy demands of the human population. The diurnal
and seasonal cycle of the solar radiation access may be one of reasons, even though the
equally random source, wind energy, is greatly subsidized in EU countries.
While the energy density of the wind power does not exceed 10 W/m² land area (Hayden
2001) and the accessibility generally does not come up to 18-20% of the annual time (Energy
in Sweden 2008), the insolation can deliver more than 1 kW/m² (heat) or ~50 W/m²
(electricity) with the accessibility in some areas more than 40% of the annual time.
The diurnal and at latitudes above 23° North, the seasonal variations of the insolation require
storage of the solar energy in magazines with seasonal heat capacity for the whole year house
heat demand.
There are principally two modes of energy storage: the chemical and the physical one.
Quantitatively, some storage modes allow large scale capacity storage, (GWh - TWh) some
are suitable for middle storage scale (1 - 1000 MWh) and others have just small scale capacity
(less than 1 MWh).
In this article, the storage system for annual heating of houses by means of magazines using
physical properties of the storage material is described.
2.
SYSTEM PRINCIPLES
2.1. Physical storage method of the sun energy.
The thermal capacity of the physical heat reservoirs is based on thermodynamic properties of
the storage material: specific heat and/or the latent heat.
2.1.1. Sensible heat storage.
While most of common materials, usable for heat storage, have specific heat cp = 0,8 – 1,6
kJ/kg/°C, (0,23-0,44 Wh/kg/°C), cp of water substantially differs with cp = 4,19 kJ/kg/°C
(1,16 Wh/kg/°C). Heat content of any reservoir utilizing sensible heat can be calculated as
follows:
Hs = cp*Δt*m*
(1)
where m = mass of the storage volume, Δt = 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, 1m³ water 90°C warm can
theoretically release 104,7 kWh of useful heat after cooling it to 0°C.
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) at10°C 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 patent
database (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 0°C, 0,8% at 10°C
and 1,5% at 20 °C (w/w) or 1 kg H2O in approx. 130 m³ of the humid air.
The average temperature measured in Stockholm area (year 2007; 59°27´N, 17°45´E) was
8,0°C and the average relative humidity was 82,2%. Average retrievable energy in each 100
m³ air is thus at least 2800 kJ (786 Wh), if the heat retrieval system can be cooled to
temperature -5°C. 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 m³ soil, saturated to 50% with the
water 15°C warm, can release more than 60 kWh when cooled down to -0°C. 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 the
insolation and in the absence of the solar radiation it collects the heat from the air.
4. EXPERIMENTAL PART
The heating system for a small house with the ground plan 50 m² and the fit up attic was built.
It consists of a 2,7 m² open solar heat collector designed for heating of swimming pools
(Texsun), heat pump (Megatherm) and a heat storage space 32 m³ containing humid soil. The
heat absorbing surface of the SHC is protected in a distance of 10 cm on the front side with
the transparent plastic sheet and on the back side with plywood board. The lower and upper
parts of the SHC are opened, so that the air can freely flow or is forced to flow in the
interstitial space by an electrical fan.
The heat pump, type GvP-25 has an effect 2,5 kW and warms the floor of the house. The heat
absorbing loop of the heat pump (60 m long copper tube 10 mm i.d.), situated at the bottom of
the pit of the size 4 x 8 x 1 m, is connected in series with a flat heat exchanger. The heat
exchanger is placed between the throttle valve of the heat pump and the copper tube. The
pattern of the tube is depicted in figure one (loop 5) and has the distance between the parallel
sections of the tube ca 40 cm.
The flat heat exchanger mediates the heat transfer between the SHC circuit containing brine
and the working fluid of the heat pump.
The pit is filled with the excavated soil to the half of the depth and a second tubing system is
laid at this level. It consists of two 32 mm diameter polyethylene tubes, situated along the
long sides of the pit. These tubes are connected with 20 parallel tubes 16 mm i.d. (figure one,
loop 9). This tubing system is used for warming the soil in the pit with excess heat from the
SHC. 20 cm above this pipe system is situated drainage pipe system with a pattern similar to
the previous one (loop 12 in the figure one). The pit is then filled to the full height of 1 m with
the soil. The drainage pipe system distributes the rain water collected from the roof of the
house.
Temperatures at critical parts of the system, solar insolation and relative humidity of the air
are measured every third minute. Five measurements are averaged and the results are stored in
a data logger (EMS). 10 m from short side of the pit, 1 m below the northern part of the
house, the sensor measuring the natural variations of the soil temperature is placed.
Temperatures are measured with Pt 100 sensors. Solar irradiation is measured with the
pyranometer (Kipp & Zonen, model CM3 ) and the relative humidity with the humidity
sensor (EMS 33)
4.1 Function description of the system.
Figure 1: Scheme of the heating system (patent applied for).
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 3°C, the brine circulates between 1, P ad B ports of the
valve 2, 7a, 9, 11, 10, 4 and 1.The soil in the pit is heated. When the temperature difference
falls to less than 2°C, 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 -5°C. 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; R² = 0,9572
(2)
18
Sept 08 - Feb 09
16
y = -0,6772x + 14,248
R2 = 0,9572
kWh/day
14
12
10
8
Temp °C
6
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
Figure 2: Measured thermal Properties of a House at the Latitude 59°27´ N
The measured mean atmospheric temperature in the year 2008 at the house site was 8,01°C.
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.
Table 1: Energy Distribution during the Year (kWh).
Sum Air+Sun kWh/5m²
Jan
-852
415
139
620
1173
Feb March
-755 -816
1170 2053
117
98
634
602
1921 2753
Month
Energy Demand kWh
SUN kWh/5m²
Energy in the Moisture
Energy in the Air
Sum Air+Sun kWh/5m²
Jul
-213
3265
446
2161
5872
Aug
-280
2092
507
1880
4480
Energy Demand kWh
SUN kWh/5m²
Energy in the Moisture
Energy in the Air
Sept
-400
1678
332
1398
3408
Apr
-500
2681
150
1087
3919
Oct
-535
1207
281
1148
2637
May
-205
3074
216
1582
4872
Nov
-765
631
160
668
1459
June
-227
3195
319
1918
5432
Dec
-949
308
123
524
954
Sum
6500
21770
2887
14224
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 m² and -5°C cold surface.
The air volume passing the collector during each night is calculated to 500m³. 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 7,2°C and the water content of the soil was 65%. The calculated
nominal heat capacity of the pit was 67 kWh/m³, 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
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 -1,5°C 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 dropped
the temperature in the pit to lowest value of the season, -4,3°C simultaneously with the
measured lowest temperature of the air, -10,9°C . 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 collecting
the 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/ m³ of
utilizable energy. And the useful energy density can be >100 kWh/ m² 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 2008
http://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.se
www.megatherm.se
EMS Brno, Czech Republic; model VV. www.emsbrno.cz
www.kippzonen.com
EMS33 from EMS Brno, Czech Republic; www.emsbrno.cz.