Труды Всемирного конгресса Международного общества солнечной энергии - 2007. Том 2
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Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
922
T
T
T
Fig. 1: Schematic of a space heating system.
2.2 The Main Components of the System
(1) Collectors: In order to reduce heat loss, evacuated
tube collectors are employed in the system. Total aperture
area is 30 m
2
. The collectors are positioned east-west
and oriented to due south.
(2) Storage tank: The volume of the tank is 4.0m
3
for the
basic case.
(3) Auxilliary heater: When the solar power is not enough,
a auxiliary heater is required to heat the supply hot water to
required tmperature if the inlet temperature is less than
50
o
C.
2.3 Description of the House
The house is located in Beijing.
The size and shape is:
Length of south/north face: 10 m
Length of west/east face: 10 m
Height: 3.3m
Window: in south face, 9m
2
The 4 walls are exposed to ambient, its ceiling and floor are
adjacent to the internal zone of same temperature as the
room in question.
The exterior walls are made of 50 mm insualtion with 200
mm heavyweight concrete. The floors and ceilings are
made of 200 mm heavywieht concrete.
2.4 Operation Of The System
In typical continuous space heating systems, the load
pumps run continuous with constant flow rates. In the
time-scheduled space heating system discussed in this
paper, the system begin to run before occupation of the
building and stop running before the end of occupation.
Typically, it is in operation during daytime and is stopped
during night.
3. ROOM MODELS
The room model is essential for the simulation, so it will
be described here in brief. The room model includes
walls, windows, roofs/ceilings, doors, and floors. The
heat transfer from and within the zone are formulated in
a matrix and solved in different ways for different type
of envelopes. The Mathematical description is as
follows
[1]
.
3.1 Exterior Wall
The instantaneous heat flux entering or leaving the zone for
an exterior wall can be modeled according to the following
transfer function relationship.
,,, ,,, ,,
001
i hi saih hi eqih hi ih
hhh
qbT cT dq
== =
=−−
∑∑∑
(1)
The coefficients
hb , hc and hd are transfer function
coefficients for current and previous values of the solid-air
temperature (
T ), equivalent zone temperature (
T
),
and heat flux,
q . A value of h equal to zero represents the
current time interval, h equal to one is the previous hour
and so on. The solid-air temperature,
T , is the
temperature of the outdoor air which, in the absence of all
radiation exchanges, would give the same heats transfer at
the outside surface as actually occurs.
3.2 Window
The solar energy passing through the window is the product
of the incident solar radiation and the transmittance
provided as an input. The thermal conduction through the
window from the ambient is given as
()QAU TT=− (2)
3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS
923
3.3 Radiative Gains
Radiation gains to each surface in the room originate from
lights, people, and solar radiation entering windows. Solar
radiation passing through windows is assumed to be
diffusely reflected.
3.4 Internal Space
An energy balance on the zone air plus any furnishings
considered as a lumped system yields
()
TT
Cap h A T T Q Q
t
−
=−++
Δ
∑
(3)
T
ZI
, T
ZF
: Room temperature at the initial and final point of a
time step;
Cap: Capacitance of Room air and furniture;
The first term of right side of the equation refers to heat
transfer between room air and inner surface of a wall,
partition, etc. The second term refers to heat gain by
infiltration. The third term refers to heat input by air
heaters.
4. SIMULATION AND DISCUSSIONS
In order to study the characteristics and performance of
solar space heating systems ˈ hourly simulations are
conducted on the basic case described in the above section.
The hourly data during the January of typical
meteorological year (TMY) comes from [2].
4.1 Standard Case
Case0 is the standard case, the system is described in the
above section and the space heating runs continuously even
in the nights. The room temperature profile during the first
week in January is shown in Fig. 2 and some indice of
performance are shown in Table 1. The solar fraction is
really high and it is due to the good solar resources in
Beijing.
7,0(K
5RRP7HPSHUDWXUH&
Fig. 2: Room temperature profile for case 0.
TABLE 1: KEY INDICE FOR THE CASE 0-3
Cases Qload Qaux Qsolar η f
GJ GJ GJ % %
Case0 3.354 0.718 2.636 25.7 78.6
Case1 2.636 1.07 1.566 21.0 59.4
Case2 3.116 1.479 1.637 21.2 47.5
Case3 2.604 0.427 2.177 24.3 83.6
Note:
Qload: Heat supplied to the room
Qaux: Heat produced by auxilliary heater
Qsolar: Useful heat gain by collectors
η : Average collector efficiency
f : Solar fraction in total
4.2 Using Time Scheduled Heating
Fig. 3 shows the room temperature profile in January by
just stopping space heating during nights. The daily average
temperature drop day by day until it finds a new balance
point. It is impossible to keep the room temperature as high
as before if we just stop space heating during nights and do
7,0(K
5RRP7HPSHUDWXUH&
Fig. 3: Room temperature profile in January.
Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
924
not change all the other things. Therefore we must take
some measures to compensate for heat input reduction
resulting from no space heating during nights. There are
several ways to deal with this.
(1) Case 1: Raising Supply Water Temperature
By several trials, we find the daily average temperature can
keep stable by raising supply water temperature to 70
o
C.
The room temperature fluctuate widely than continuous
heating, but the fluctuation during daytime is adequately
acceptable. This is indicated by case1 in Table 1. The heat
load is really reduced but the auxilliary heat is increased.
This means that this case will not save convectional energy
but use more.
(2) Case 2: Start space heating before occupation
In this case, space heating begins at 5:00 (3 hours before
occupation) and supply temperature is reduced from 75
o
C
at 5:00 to 45
o
C at 8:00. The room temperature variation
during daytime is mitigated than case1. However, both
heat load and auxiliary heat are greater than case1. It can
not save energy compared to case0.
(3) Case3: Increasing heat transfer area and flowrate
In this case, both the heat transfer area and flowrate are
doubled compared to case0, the others are kept unchanged.
The room temperature profile is similar to case1. From
table 1, we find that both the heat load and auxilliary heat
are reduced. This means that it really can save energy.
4.3 Using Time Scheduled Heating to Rooms of Light
Weight Walls
We also studied the effects of using time scheduled heating
to rooms of light weight walls. In cases 4 and 5, the exterior
walls are made of 50 mm insulation with 200 mm light
weight concrete blocks, which are only one forth of the
heavy weight concrete in weight but its thermal resistance
are much larger than the former.
Case4 employs continuous heating and case 5 using time
scheduled heating companied with doubled heat transfer
area and flowrate. The results are similar to room of heavy
weight walls.
TABLE 2: KEY INDICE FOR THE CASE 4-5
Cases Qload Qaux Qsolar η f
GJ GJ GJ % %
Case4 2.628 0.684 1.944 26.3 74
Case 5 2.143 0.490 1.653 24.8 77
4.4 The Effect of Collector Areas
This paper also conduted comparison between continuous
heating and time scheduled heating using the same control
strategy as case 2 for different collector areas. It is shown
in Fig. 4. it shows that time scheduled heating companied
with higher-supply-temperature heating in advance uses
more conventional energy than continuous heating in solar
space heating systems. The greater collector area, the more
conventional energy will be consumed due to stopping
heating during nights.
&ROOHFWRU$U HDP
$X[LOOLDU\+HDW
0-
&RQWLQXRXVP RGH 'LIIHUHQFHE HWZHHQWZRPRGHV
Fig. 4: Comparison between continuous heating and time
scheduled heating for different collector areas.
5. CONCLUSIONS
(1) Using time scheduled heating (stopping heating during
nights) can reduce the heat load compared to continuous
heating.
(2) It is impossible to keep the room temperature as high as
before if normal systems of continuous heating are changed
to time scheduled heating mode ( stopping space heating
during nights and not changing all the other things).
Some measures must be taken to compensate for heat
input reduction resulting from no space heating during
3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS
925
nights. The measures include raising supply water
temperature, increasing heat transfer area and flowrate and
heating in early morning before occupation.
(3) Whether time scheduled heating in solar space heating
can save conventional auxilliary energy compared to
continuous heating depends on the control strategies. When
companied with raised supply water temperature, it uses
more conventional auxilliary energy and can not save
energy. When using more heat transfer area and higher
flowrate, it can save auxilliary energy.
6. ACKNOWLEDGEMENTS
The research is financed by Shanghai Education Committee
Development Foundation (06EZ005).
7. REFERENCES
(1) Klein, S.A., W.A. Beckman, and J.A. Duffie. 1996. A
transient simulation program-Reference manual.
University of Wisconsin-Madison. (USA).
(2) China Weather Bureau, TsingHua University. 2005.
Special databse for building thermal environment in
China. China Architecture & Building Press.
(China).
TWO DEMONSTRATIONS OF SOLAR HEATING AND AIR-CONDITIONING
SYSTEM IN BUILDINGS IN CHINA
Zhang Xinyu, Zheng Ruicheng, Feng Xiaomei , Zou Yu, He Tao ,Xuwei ,Zhang JianghuaˈLi Zhong
China Academy of Building Research
No.30,Beisanhuandonglu
Beijing 100013, China
zxyhit@163.com
ABSTRACT
Two demonstrations of solar air-conditioning and heating
system in office building are introduced, one is solar
heating system with seasonal storage, flat plate solar
collector integrated into external wall, the solar fraction for
space heating is 25%, another is solar-assisted
air-conditioning system, evacuated tube solar collector
integrated with flat roof, the solar fraction for space heating
is 20%, for air-conditioning is 50%. The auxiliary energy
equipment is ground-source heat pump system. The control
strategy for solar system and auxiliary energy system are
also introduced. The project is financial by National
Renewable Energy Fund of China.
1. INTRODUCTION
The demonstration project located in Tongzhou District of
Beijing, near to Beijing Capital International Airport. The
project invested by China Academy of Building Research,
is the national demonstration project for energy
efficiency in building on city level. The total construction
area of the project is 9460 , divided into two parts. One is
2835 ,named South Part, another is 6625 ,named North
Part.The flat plate solar collectors integrated into external
wall with seasonal storage are used in the South part . The
evacuated solar collectors on the flat roof are used in the
North Part . The function of the building in project is office
and testing laboratory.
2. BUILDING DESCRIPTION
Solar energy system and ground-source energy system in
this project supplied the energy for cooling and heating.
The energy consumption of the building must be a lower
level. The basic information of building is following,
the average ratio of window to wall is 0.23;
the U-value of the external wall is 0.4 W/(m
2
gK)
the U-value of the roof is 0.275 W/(m
2
gK)
the U-value of the ground is 0.25 W/(m
2
gK)
the U-value of the windows is 2.0 W/(m
2
gK)
the U-value of the door is 1.8 W/(m
2
gK)
According to the simulation result with Trnsys, the
energy efficiency in building of this project is 65%
compared with pat public building in China and is higher
Chinese standard.
3. SYSTEM DESIGN
3.1 North Part
Solar heating system supply energy for space heating in
winter, the ground-source heat pump as the auxiliary heat
source. In the season except winter, solar system store the
heat in the concrete tank, the volume of which is 200
m
3
.In Winter , the ground-source heat pump and storage
tank meet the cooling demand together. The terminal system
3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS
927
for space heating is radiator and floor heating system. To
make full use of the energy from solar system, Firstly, the
higher temperature water from solar system pass the
radiator, then go into floor heating system.
The type of solar collector used in this part is flat plate
solar collector, integrated with external wall, it looks like
curtain wall but solar collector. The total amount of solar
collector is 140 ,the dimension of the collector is 1000
mmh2000 mm. The aim of this part is to demonstrate the
solar collector integrated type and seasonal heat storage.
Fig. 1 is the appearance of flat plate solar collector
integrated with wall.
Fig. 1: Appearance of solar collector integrated with wall.
The simulation result is that the solar fraction of this
system is 25% for space heating.
3.2 South Part
Solar heating system supply energy for space heating in
winter and supply heat to drive the absorption to produce
cooling water for air-conditioning in summer. The
ground-source heat pump as the auxiliary source.
The type of solar collector used in this part is evacuated
tube solar collector, integrated with flat roof. The total
amount of solar collector is 228.48m
2
,the dimension of the
collector is 1020 mmh2000 mm. The aim of this part is
to demonstrate the solar –assisted air-conditioning system.
Fig. 2 is the appearance of evacuated tube solar collector on
the roof .
Fig. 2: Appearance of solar collector on the roof.
In this system, two water tanks are used, one is for heat
water storage, the volume of which is 10m
3
, another is for
cooling water storage, the volume of which is 15m
3
.The
output of the refrigeration is 105 kW.
The simulation result is that the solar fraction of this
system is 20% for space heating and 50% for cooling. In
this part, the space heating building area is more than
cooling.
4. SYSTEM CONTROL STRATEGY
The system must take different control strategy in different
season. The detail description of control strategy is following.
4.1 North Part
In the heating season, solar heating system and
ground-source heat pump system supply energy for space
heating together. In summer, the ground-source heat pump
system meet cooling demand, in other season the solar
collector system stored the heat in concrete tank for space
heating in Winter.
4.1.1 Heating Season
The solar collector system is controlled by temperature
difference between the highest temperature part of the
solar collector and the lowest temperature in the concrete
tank. When the temperature difference bigger than 5ć, the
Flat plate solar collector
Evacuated tube solar collector
Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
928
pump of solar collector system started, till the temperature
difference lower to 2ć,the pump stopped. The freeze
protection of the system is drain back system, while the
pump of solar collector system stopped, the water in solar
collectors and pipes drained back into concrete tank.
When the water temperature in the concrete tank is more
than 50ć, the solar heating system directly supply energy
into building for space heating. When the temperature of
the return water from the heating system is higher than 45
ćand lower than 30ć, the heat pump heat the water into
required temperature to meet the heat demand. When return
temperature is lower than 30ć, the solar collector system
store the heat in the concrete tank.
4.1.2 Summer
The ground-source heat pump supply cool water for air
conditioning. The solar collector system is controlled with
the same in heating season. The solar collector system store
the heat in the concrete tank for space heating in Winter,
when the temperature in the concrete tank is higher than
95 ć, the pump of solar collector system stopped.
4.1.3 Other Season
The solar collector system is controlled with the same in
heating season. The solar collector system store the heat in
the concrete tank for space heating in Winter , when the
temperature in the concrete tank is higher than 95 ć, the
pump of solar collector system stopped.
4.2 South Part
In the heating season, solar heating system and
ground-source heat pump system supply energy for space
heating together. In Summer, the solar heating system
supply the heat to absorption refrigerator to cool water for
air conditioning, when the radiation is not enough to
produce higher temperature water, the ground-source heat
pump system started.
4.2.1 Heating Season
The control strategy of solar system is the same with the
North Part in heating season. The freeze protection of the
system is anti-freeze circulation system, when the
temperature in the top part of solar collector system is
lower than 5ć, the pump of solar collector system started,
when temperature increased to 10 ć,the pump system
stopped, part of water in solar collectors and pipes drained
back into heat storage tank.
When the water temperature in the heat storage tank is
more than 50ć, the solar heating system directly supply
energy into building for space heating. When the
temperature of the return water from the heating system is
higher than 45ćand lower than 30ć, the heat pump heats
the water into required temperature to meet the heat
demand. When return temperature is lower than 30ć, the
water heated by solar system go into the evaporator of the
heat pump to improve the efficiency of heat pump for
saving more conventional energy such as electrical power.
4.2.2 Summer
The solar collector system is controlled with the same in
heating season. When the temperature in heat store tank is
more than 88ć,the pump start ,and the absorption refrigerator
started to cool the water in the cooling storage tank.
When the temperature in the top part of cooling storage
tank is lower than the return water temperature, the pump for
supply cooling started. The solar assisted air- conditioning
system and the ground heat pump are parallel operating.
When the return water temperature is lower than the
temperature in the bottom part of cooling storage tank, this
means the cooling demand is less.
Firstly, the absorption refrigeration system cools the water
in the cooling storage tank, when the temperature in which
meet the design requirement, the absorption refrigeration
system stopped.
Then, the solar collector system heat the water in the heat
storage tank. When the temperature in the heat storage tank is
higher than 95ć, the pump for solar collector system stopped.
4.2.3 Other Season
In spring, the solar system and ground pump system supply
3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS
929
heat for space heating till the building dose not need
space heating. Firstly, the absorption refrigeration
system cools the water in the cooling storage tank, when
the temperature in which meet the design requirement,
the absorption refrigeration system stopped. Then, the
solar collector system heat the water in the heat storage
tank. When the temperature in the heat storage tank is
higher than 95 ć, the pump for solar collector system
stopped.
In autumn, the solar system and ground pump system supply
heat for cooling till the building dose not need cooling. the
solar collector system heat the water in the heat storage tank.
When the temperature in the heat storage tank is higher than
95 ć, the pump for solar collector system stopped.
5. PROJECT MONTIORING
The solar system will be completed in July,2007.The
monitoring equipment installed in the system will be used to
test the basic parameters such as temperature, flow, radiation,
power consumption and etc. The aim for monitoring is to
improve the design idea. From the monitoring data, the
solar fraction of system, the system efficiency and other
index can be acquired. Through the monitoring, much
experience can be accumulated to better promote the solar
energy application in building.
6. REFERENCES
(1) Hans M. H, “Solar-assisted air-conditioning in
buildings”, New York: Springder Wien New Yirk,
2004
(2) Zheng Ruicheng, “Technical Guidebook for Solar
Water Heating System of Civil Buildings”, Beijing:
Chemical Industry Press,2006
(3)
China Nonferrous Engineering and Research Institute,
GB 50019-2003 Code for Design of heating, ventilation
and air condition. Beijing: China Planning Press, 2004
(4) China Academy of Building Research,GB50366-2005
Technical Code for Ground-source Heat Pump System.
Beijing: China Architecture & Building Press,2005
STUDIES OF A THERMOSYPHON SYSTEM WITH A HEAT SOURCE NEAR
THE TOP AND HEAT SINK AT THE BOTTOM
Sadasuke Ito
Kanagawa Institute of Technology
Atsugi, 243-0292 Japan
E-mail: ito@sd.kanagawa-it.ac.jp
Kenichi Tateishi
Kanagawa Institute of Technology
Atsugi, 243-0292 Japan
Naokatsu Miura
Kanagawa Institute of Technology
Atsugi, 243-0292 Japan
ABSTRACT
Thermosyphon are heat transport devices that can transfer
heat using gravitation. They can work without any external
power supply, and have simple structures. Recently, a
device which transferred heat from the hot reservoir near
the top to the cold reservoir at the bottom was invented by
Ippohshi et al.[1] In this study the same type of device was
made and the performance was examined. Then, another
type of device which was simpler to make was proposed
and the performance was compared with each other. It was
found that the one proposed in this study did not take time
before the water in the tube circulated and that there was a
possibility for applying the device to a solar water heating
system which would work without a pump.
1. INTRODUCTION
There are mainly two types of systems for solar water
heaters. One is an active type which uses a pump to
circulate the working fluid such as water and polypropylene
glycol through collectors. To operate the system
appropriately, a pump and a control system for circulating
the fluid are necessary. The other type is a passive or
thermosyphon system which circulates the working fluid
naturally by the effect of the gravitational force acting on
the fluid. This system has a great advantage but the storage
tank must be above the collectors in order to circulate the
fluid naturally by thermosyphon. When a thermosyphon
system is set on a roof of a house, it weighs the roof and the
appearance of it would not be good. Morrison[2] digested
problems which had encountered in thermosyphon systems
in solar water heating. If a lot of heat can be transferred
from a heat source at a level to a heat sink at a lower level
naturally, the method would be able to be applied not only
to solar water heaters but also many other fields such as
space heating, road heating for melting snow, and
distillation using solar heat.
Maydanik[3] reported about a loop heat pipe which
equipped with an evaporator in which a wick sintered from
fine-grained particle to produce capillary pressure for
circulating the working fluid in the loop. The vapor
evaporated in the evaporator was condensed at the
condenser where heat was extracted. The loop heat pipes
had been most extensively employed in thermoregulation
devises. In the paper, ideas to use the devices for solar
collectors were described.
Ippohshi et al.[1] developed a unique top-heat loop
thermosyphon which used the gravitational force to transfer
heat from a heat source at a level to a heat sink at a lower
level. This system could operate successfully achieving
high heat transport rate except in the beginning of operation.
3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS
931
This system used a rather big condenser, in which there was
a coil of a tube, above the evaporator and the system
seemed a little complicated. In this study, we modified the
system proposed by Ippohshi et al.[1] to make the system
simpler and the performance of the two systems was
compared .
2. EXPERIMENT
A model of the top-heat loop thermosyphon proposed by
Ipposhi and et al.[1] Apparatus No. 1, is shown in Fig. 1(a).
The evaporator, condenser (heat exchanger) and cooler are
connected by a tube. The condenser works also as the
reservoir of the working fluid. The working fluid in the
loop is supplied after evacuation so that there are only the
saturated liquid and the saturated vapor in the condenser.
When the working fluid is heated at the evaporator, the
liquid evaporates and vapor is formed in the evaporator.
Since the density of the mixture is smaller than the liquid in
the other parts of the loop, a gravitational force is produced
to circulate the working fluid in the direction shown by the
arrows in the loop. If there is a flow in the loop, the vapor
in the condenser is condensed on the surface of the tube in
which the liquid chilled by the cooler flows. The liquid
formed by condensation from the vapor flows to the outlet
at the bottom of the condenser. The flow in the tube in the
condenser is heated by the latent heat of condensation and
the sensible heat of the saturated liquid in the condenser
before it leaves for the inlet of the evaporator. The
Apparatus No. 1 for the experiments is shown in Fig. 1(b).
Water was used as the working fluid.
A model of the top-heat loop thermosyphon proposed in the
present study, Apparatus No.2, is shown in Fig. 2(a). The
system equipped with a small reservoir of water at the end
of the condenser as shown in the figure. The condenser
located on the top of the system is consisted of two tubes
soldered together. The flow of the mixture of the liquid and
vapor in the upper tube is cooled by the lower tube in
which the liquid from the cooler flows so that the vapor in
the upper tube is condensed. When the volume of the vapor
is increased, the total volume of the vapor and the liquid
expands. It is aimed to have reservoir for storing extra
liquid in the loop.
(a) Model (b) Experiment
Fig. 1: Schematic of Apparatus No. 1.
(a) Model (b) Experiment
Fig. 2: Schematic of Apparatus No. 2.
The Apparatus No. 2 for the experiments is shown in Fig.
2(b). The inside and outside diameters of the loop tube
which is made from copper are 8.0mm and 9.5mm,
respectively except the tube for the evaporator (heater). The
same tube is used in the Apparatus No. 1. The inside and
outside diameters of the copper tube for the evaporator are
11.0mm and 12.7 mm, respectively. The length of the
evaporator is 600mm. The length of the tube of the
condenser is 2.7m for the apparatus No. 1 and 1.8m for the
Apparatus No. 2. The length of the coil dipped in the cooler
is 12m for each apparatus. The length between the centers
of the evaporator and the cooler are 1460mm in Apparatus
No. 1 and 1000mm in Apparatus No. 2. The evaporator of