High performance passive solar heating system with heat pipe
energy transfer and latent heat storage modules
Citation for published version (APA):
Dijk, van, H. A. L., Brink, van den, G. J., Hensen, J. L. M., & Raamt, van, A. (1985). High performance passive solar heating system with heat pipe energy transfer and latent heat storage modules. (EUR; Vol. 9720). Commission of the European Communities.
Document status and date: Published: 01/01/1985
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Commission of the European Communities
energy
HIGH PERFORMANCE
PASSIVE SOLAR HEATING SYSTEM
WITH HEATPIPE ENERGY TRANSFER
AND LATENT HEAT STORAGE MODULES
Report
EUR 9720 EN
Commission of the European Communities
energy
HIGH PERFORMANCE
PASSIVE SOLAR HEATING SYSTEM
WITH HEATPIPE ENERGY TRANSFER
AND LATENT HEAT STORAGE MODULES
H.A.L. VAN DIJK, G.J. VAN DEN BRINK, J.L.M. HENSEN, A. VAN RAAMT
TECHNISCH PHYSISCHE DIENST TNO-TH Stieltjesweg 1
Postbus 155
2600 AD Delft - The Netherlands
Contract No. ESA-PS-140NL FINAL REPORT
Directorate-General for Science, Research and Development
Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES Directorate-General
Information Market and innovation Bâtiment Jean Monnet
LUXEMBOURG
LEGAL NOTICE
Neither tne Commission of the European Communities norany person acting on behaif o: the Commission is responsive for the use which might be made of the following
information
TP D technisch physische dienst tno-th
adres Stieltjesweg t 2628 CK Delti postadres Postbus 155 2 2 6 . 2 0 5 Ha 2600 AD Délit telefoon ( 0 1 5 ) 7 8 8 0 20 telex 38091 tod dt niHIGH PERFORMANCE PASSIVE SOLAR HEATING SYSTEM WITH HEATPIPE ENERGY TRANSFER AND LATENT HEAT STORAGE MODULES
ABSTRACT
Results are reported from a project on the development of a high performance passive solar heating system.
Three components have been examined in detail by series of measurements:
a. a heat pipe as a thermal diode tube for the efficient transfer of collected solar heat from the absorber plate to behind an insulation layer.
b. a naturally ventilated air cavity for the transfer of stored heat to the room.
c. a heat storage with high storage capacity at moderate operating temperatures.
The results of these component studies were used for the design of the 3olar system.
A detailed constructive design was made for extensive manufacturing cost analyses. The expected manufacturing costs of the system appeared to be too high for a reasonable pay-back period. However, because, only a small portion of this price consists of material costs, it is foreseen that other, less complicated, design principles can make the system much cheaper.
An extensive parameter study with a unsteady state model of the h.p. system in a room showed, that the designed system has a high annual performance. The system appeared to be very superior over a conventional storage wall. Moreover, the parameter study showed, that it is possible to maintain the high performance with less complicated design principles, thus enabling cheaper production. The calculations also showed, that the performance can further be increased significantly by improving a few details in the design.
This project has been financed by the Commission of the European Communities and by the Dutch National Solar Energy Programme.
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SUMMARY
Within the passive solar approach, the direct gain and convective loop systems are the most simple and therefore cheapest solutions. These kinds of systems, however, are characterised by high indoor temperature swings and high solar heat supply on the hours of least demand.
A thermal storage wall has an advantageous accumulation of solar heat which results in low temperature and a time delay between the absorption of solar energy and the heat supply to the building. The main disadvantage of a conventional thermal storage wall, however, is the higher temperature of the solar collecting surface which can lead to considerable heat loss to the outside. Multi-glazing can limit this heat loss to a certain extent, but is costly. Movable insulation is also expensive, has a slow response and needs careful attention for a reliable and effective operation.
This project is aimed at developing a high performance passive solar heating system which has not the disadvantages as mentioned above, this was to be achieved by the introduction of two special components:
a. A thermal diode tube, which acts with small temperature difference already as a high performance thermal conductor. It transfers heat from the collector to behind an insulation layer, but does not transfer heat in the reverse direction. With a low capacity collector the system responds rapidly to changes in outdoor conditions, while needing neither movable parts nor manually or automatically operated control equipment.
b. The heat storage. The significant decrease in volume of 50 per-cents can be achieved by using water instead of concrete. A
further decrease in volume, which is strongly dependent on the temperature range over which the store shall operate, might be reached by using a phase change material.
I r U nummer 2 2 6 2 0 5 ^'ad V^
The design of the passive solar element is based on the following successive components:
- glass cover - small air gap - absorber plate
- Z-shaped heat pipes through an insulation layer
- a back plate between the diode tubes and the (latent) heat storage elements
- a naturally ventilated air cavity with closable openings to separate the system from the room.
From preliminary calculations of the systems' performance and comparison with a conventional passive system, the HP passive system appeared to be very superior over a conventional storage wall, even in case the latter is assumed to operate under ideal conditions concerning heat demand and inhabitants attention. Comparison with a similar active system confirmed the high performance of the passive design.
One major part of the research was the optimization and testing of the innovative components, based on the preliminary conditions and the criteria developed at an earlier stage of the project:
a. measurement series on heat pipes, especially developed for the solar system
b. measurement series on the heat transfer in a naturally ventilated air cavity
c. measurements and calculations on heat storage materials with heat transfer to an air cavity.
The major results were:
a. a simple evacuated copper tube partially filled with water gives the best results under given conditions. The efficiency in heat transfer is high.
b. fairly generally valid equations could be derived which show acceptable thermal resistance of the cavity, which can further be improved by careful design.
I K U nummer 2 2 6 . 2 0 5 b l a d V I 1
c. calculations showed that the storage has to be connected directly to the heat pipes to have the highest performance.
The measurements could be used to validate predicted performance of a few types of materials. Storage with low operating temperatures showed a too long time (10 hours) needed to extract the accumulated heat.
A further step was to develop a detailed constructive design for an extensive manufacturing cost analysis. For the constructive design and the cost calculations B.V. Koninklijke Maatschappij "De Schelde" was involved in the project.
The aim of this part of the research project was to develop a constructive design of a prototype high performance passive solar heating system which could be produced at reasonable costs.
First preliminary designs appeared to be too complicated and therefore much too expensive.
The cost-breakdown for the final design of the prototype showed material costs as low as expected in the first stage of the project:
about Df 1. 300,— for all the materials used per nr- of collector area.
However, the net manufacturing costs still appeared to become more than three times as high, because of the addition of labour costs for assembling the element and for finishing to achieve a product which will be accepted by the consumers.
Furthermore, additional costs (a.o. VAT) lead to an all-in consumers' price again about 50% higher than the net manufacturing costs. The result is a prototype h.p. passive solar system with a price of about Dfl. 1550/m^ collector. For this price the element can be delivered ready for use mounted in the facade.
A price of Dfl. 350,— would have resulted in a estimated pay-back period of approximately 15 years. This implies that the design is still too complicated.
The results from the last step in the project, the study on the thermal performance of the system, are needed to make clear whether it is possible to use other design principles which would make the system cheaper.
I r U nummer joe 20.5 b'ac* VIII
The details which need attention from the constructors' point of view are:
- the connection of heat pipes to absorber plate - the connection of heat pipes to storage
- the number of heat pipes
- the storage container, shape and material - the venting openings.
Other options to decrease the manufacturing costs are a system without storage or the use of an electric fan to increase the performance.
Simultaneously with the cost analyses an extensive parameter study has been performed with an unsteady state model of the high performance passive solar system, under realistic climatic conditions and with realistic hourly heat demand, indoor air and wall temperatures for a few types of dwellings and occupants' behaviour, for typical Dutch circumstances:
- The characteristics of the h.p. passive element in the reference case have been derived from the previous investigations concerning the major components of the system (prototype design)
- The net heat gain has been defined as the useful heat per heating season from 1 m^ of the system to the room, increased with the heat loss through 1 r of a wall which has been replaced by the solar system; the U-value of this wall being adapted to the insulation level of the dwelling under consideration. The useful heat is that part of the total (positive and negative) heat flow through the system that can be used to deminish the heat demand of the room. - From these calculations the following general conclusions could be
derived:
. The net heat gain of the system amounts about 130 kWh/m2 to 190
kWh/m2 per heating season, depending on variations in design,
heat demand of the dwelling and occupants' behaviour.
. If the system is built according to the prototype design as previously developed during the project the net heat gain amounts about 130 kWh/m2.
TPD
nummer 226 205 blad IX. The heat gain can be increased significantly by improving the so-called "fin factor" of the system, the resistance between the surface of the absorber plate and the heat pipes and the resistance between heat pipes and storage section.
Depending on the efficiency of this improvement the net heat gain can in this way be increased to 160 or 190 kWh/m2 as calculations
indicated, e.g. by increasing the absorber plate thickness and the number of heat pipes per m2. The latter measure, however,
could be replaced by more cost-effective means, e.g. longer heat pipe ends, to achieve a better coverage of the absorber plate. . A decrease in the heat pipe resistance itself has only a minor
positive effect; 5 heat pipes/m2 therefore seems sufficient,
provided the fin factor has been improved.
. The heat gain can also be increased by decreasing the cavity thermal resistance e.g. by adding extra heat exchange surface. This improvement may add about 15 to 20 kWh/m2 to the gains. An
important reduction could be achieved by forced ventilation. However, the extra gain (30 to 40 kWh/m^ compared to the reference case), will probably not compensate for the electrical energy needed for the ventilator.
. The choice of storage material type is in all cases of minor importance. The difference in heat gain with water, paraffine and salt hydrates (with various transition temperatures) are small. . In cases with relatively high heat demand (moderately insulated
dwelling, no daytime temperature set back, only night-time temperature set-back) the influence of the other storage/cavity parameters is also small:
a. the cavity needs no insulation to the room (a thin board is sufficient)
b. the storage volume can even be reduced to zero without significant decrease in performance (only -5 kWh/m2).
. In case of a highly insulated dwelling the presence of some storage is however essential. The net heat gain is then about 15 to 20 kWh/m2 less than for the moderately insulated dwelling, but
this figure includes the lower value for the heat loss (U-value) from the wall which has been replaced by the solar system.
rPD nummer 226.205 blad X
. In case of a highly insulated dwelling with daytime temperature
set-back the presence of storage is again essential, as could be
expected.
a. With storage, the net heat gain is again about 15 kWh/m
2less
than the previous case without the daytime set-back.
b. In this case however it is also essential to insulate the
cavity with a e.g. 5 cm insulation sheet.
c. The control of the cavity air flow is also important to
prevent heat flow to the room on hours without heat demand.
d. The volume of the storage however, is again not critical for
the performance. About 30 l/m
2system seems to be sufficient:
the most important criterion here is to prevent over-heating
of the storage in case of a too small volume.
. An increase of the total area of the element in the facade e.g.
from 2 to 4 rrr has no major negative effect on the gains per m
2.
These results from the performance calculations make it indeed
possible to review the principles of the constructive design, in
order to realize a much less expensive construction. The following
components need attention in this respect:
- the storage material, e.g. a water tank instead of salt hydrate or
paraffine in cylinders
- the connection of the heat pipe ends to the absorber plate e.g.
elongation of the copper tubes
- the connection of heat pipe ends to the storage material, e.g. the
ends directly into the water
T P D nummer 226.205 blad X I
Because of a possible application on short notice, simultaneously with the finalisation of this project, alternative designs to
simplify the construction and manufacturing have been taken under consideration.
Although these activities are beyond the scope of this research study, they show interesting new possibilities in case the final conclusions from this research are taken into account.
At this moment a construction is under consideration in which the back plate between heat pipe ends and storage is omitted and the heat pipe ends are directly put into the water of the storage.
A first conclusion is that with this option the cost-benefit ratio can be improved drasticly.
A second option is a diode without heat pipes. Solar radiation is captured by absorption and storage after passing a translucent insulator, with low losses to the outside surrounding. This option will be considered in a later stage.
This project has been financed by the Commission of the European Communities and by the Dutch National Solar Energy Programme. In this project the following organisations have been co-operating in the research: the Institute of Applied Physics TNO-TH, the Technical University Eindhoven and the B.V. Koninklijke Maatschappij "De Scheide".
XIII CONTENTS
1. Introduction 1 2. Aim of the project 3
3. System description 5
4. Methods 7 5. Requirements and global design 9
6. Optimization of the design 13
6ol Introduction 13 6.2 The heat pipes 13 6.3 Heat transfer in the air cavity 19
6.4 Heat storage 25 7. Constructive design and calculation of consumers price 31
7.1 Introduction · 31 7¡2 Constructive design 31
7.3 Cost analysis 38 7.4 Conclusions 39 8. Heat gain calculations of the h. p. passive solar heating system 43
8.1 Introduction 43 8.2 Calculations 43
8.3 Results and conclusions 46
9. Recent developments 49
Literature 51
Appendix 1 : Heat pipe measurements
(report 226.205/4, July 13, 1984) 53
Appendix 2 : Air cavity measurements
(report 226.205/5, July 1984) 109
Appendix 3 ; Heat storage measurements
(report 226.205/6, July 13, 1984) 141
Appendix 4 : Constructive design incl. costs
(report 226.205/7, September 1984) 177
Appendix 5 : Heat gain calculations
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INTRODUCTION
Within the passive solar approach, the direct gain and convective loop systems are the most simple and therefore cheapest solutions. These kinds of systems, however, are characterised by high indoor temperature swings and high solar heat supply on the hours of least demand.
A thermal storage wall has a advantageous accumulation of solar heat which results in low temperature and a time delay between the absorption of solar energy and the heat supply to the building.
The main disadvantage of a conventional thermal storage wall, however, is the higher temperature of the solar collecting surface which in combination with the high thermal capacity of the wall can lead to considerable heat loss to the outside. Multi-glazing can limit this heat loss to a certain extend, but is costly. Movable insulation is also expensive, has a slow response and needs careful attention for a reliable and effective operation. Furthermore, the extra mass introduced into the building may sometimes lead to extra costs particularly in multi-story buildings ana valuable space within the building is occupied by the system.
TPD
nummer 226 205 blad2. AIM OF THE PROJECT
This project aimed to develop a passive solar heating system which has not the disadvantages as mentioned above. This was to be achieved by the introduction of two special components:
a. A thermal diode tube, which acts with small temperature difference already as a high performance thermal conductor. It transfers heat from the collector to behind an insulation layer, but does not transfer heat in the reverse direction. With a low capacity collector the system responds rapidly to changes in outdoor conditions, while needing no movable parts or manually or automatically operated control equipment.
b. The heat storage. The significant decrease in volume of 50 per cents can be achieved by using water instead of concrete. A further decrease in volume, which is strongly dependent on the temperature range over which the store shall operate, might be reached by using a phase change material.
TPD
nummer 2 2 6 . 2 0 5 blad.3. SYSTEM DESCRIPTION
The design of the passive solar element is based on the following mechanisms (see figure 1 ) :
- glass cover (1). - Small air gap.
Absorber plate (2) to collect the solar radiation.
Z-shaped heat pipes as thermal diode tubes to transfer the absorbed heat through an insulation layer (3).
- A back plate (4) to distribute the heat from the diode tubes to the (latent) heat storage elements (5).
An air cavity is formed with a common insulation sheet (8) to separate the system from the room.
- When the air cavity is open to the room, air flows under the buoyancy force through the cavity (7), thus transferring the heat from the storage material (6) to the room. The cavity can be closed when there is no heat demand.
MOM
vntTKBi n»tt.srno« 3 ι s t ι
■\ na· STAMMT nmt (.s)
i «isonin TWf
ï τ « » « am ran ■ «sum« urn
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t «sr SUTTU DUOu το τ« RU,
■>. NflTUrøi. VExlliKTIM Of *0Ch f»\ TMlÛJûw 1HI CWflTT t iHCIrVH «SOLUTION SHEET
Figure 1 : Schematic drawing of the high performance passive solar heating element.
TPD
226.205 bladThe thermal characteristics of a heat pipe as thermal diode ("thermosyphon") can be shortly described as follows: the heat pipe is basically a closed hollow tube partially filled with a working fluidum, in equilibrium with its vapour. The tube is in inclined or vertical position. When the lower end is heated the fluid absorbs this heat by evaporation. At the - colder - upper end the vapour will condense and release the latent heat, the liquid will return to the lower end by gravity force. With reverse temperature difference the upper end will dry out and no heat transfer takes place in this element (thermal diode effect). The working fluid and the heat pipe tubing are chosen on the basis of the operating temperature and their mutual compatibility.
In the preliminary design the system had been provided with a venting possibility to the outside for summer through-ventilation, see figure 2.
n. nsating,
win c e reond i t ion.
\ \
\
vent ilat ing, summereond it ion ; necessary when over-heating of the systen must be prevented, also possible when cooling of the buil-ding is des i red .
\ \
1 i
^ =
si or.içp .
figure 2; Schematic view of the possible situations with regard to the
convective heat transfer from the storage space;
I r U nummer £26 205 b'ac* ■
4 . METHODS
The project consisted of the following phases: Phase 1. Global design:
defining the boundary conditions and criteria and preliminary design of the system.
Phase 2. Optimization and testing of the innovative components: material choice, dimensions and constructive details for: a) the heat pipes
b) the heat storage
c) the heat transfer mechanisms in the naturally ventilated air cavity.
Phase 3. Design of a prototype high performance system:
design of prototype, prediction of the heat gains and calculation of the costs for manufacturing.
Within phases 1 and 2 preliminary cost-benefit analyses were performed which made it possible to revise the design of the system according to preliminary results from calculations or measurements.
In these preliminary analyses the annual heat gains were calculated with a steady state computer model and manufacturing costs were obtained by examining step by step the manufacturing process.
Reports from these earlier stages in the project can be found e.g. in (1), (2) and (3).
In this project the following organizations have cooperated in the research:
the Institute of Applied Physics TNO-TH: . coordination of the project
. measurements on the heat pipes . measurements on the heat storage
- the group FAG0-TN0-THE at the Technical University of Eindhoven: . measurements on the air cavity ventilation
. heat g a m calculations
- 8.V, Koninklijke Maatschappij "De Scheide": . manufacturing costs calculations
TPD
nummer 2 2 6 . 2 0 5 blad5. REQUIREMENTS AND GLOBAL DESIGN
As a basis for the design of the system the following requirements were drawn up:
1. Low thermal resistance in heat transfer from absorber plate to the heat storage.
2. High thermal resistance in heat transfer from absorber plate to the outside.
3. High solar transmittance of the glass cover and high solar absorptance of the absorber.
4. Thermal resistance from heat storage to absorber plate at least as high as for a conventional well-insulated wall (R = 2.3 nr^.K/W)
5. Moderate thermal resistance between storage and room on hours without heat demand and low thermal resistance otherwise, to be controled manually or automatically.
6. Low manufacturing and maintenance costs.
7. Average conditions during heat collection from the sun: 200 W/m2 solar
irradiation and outdoor temperature 5° C.
8. Maximum solar irradiation: 1000 W/m2 on the south facade; minimum
outdoor air temperature: -10° C.
9. Application in dwellings in the south facade of the living area.
The last requirement was introduced because of the following considerations:
Dwellings have the advantage over office-buildings that surplus-heat from daytime can be stored and used in the evenings when the heat demand is usually greater. For office-buildings 3urplus-heat has to be stored until the next morning, which is much more difficult to achieve.
The facade is chosen instead of e.g. the roof because of the passive nature of the system. A system as part of the roof surface would have the advantage that the available area is high, and that there is less chance on shading from surrounding objects. Bue the disadvantage is that the collected heat cannot be transferred to the living areas by natural means. One shouic also note that the functioning of the heat pipes depends on gravity forces. This implies that an inclined system could require different design details.
I r U nummer ο?6 ?Π5 blad _ 1 0
These requirements and preliminary cost-benefit analyses led to a global design which served as a reference for the optimization study, in particular for the testing of the components.
The reference design consists of:
glass cover: one pane of untreated clear glass small airgap
absorber plate with spectral selective surface (E = 0.50) - heat pipes: 5 per m2 collector area
length of evaporation zone: 0.10 - 0.30 m. adiabatic zone: 0.20 m. condensation zone: 0.10 - 0.30 m. material: copper
diameter: outside: 6 mm. inside : 5 mm. insulation layer thickness: 0.20 m. - air cavity: 0.05 - 0.15 m.
with some extra heat exchange surface
storage elements: variable shapes and dimensions insulation sheet: variable thickness
ventilation openings: variable area - total height: 1.0 - 2.0 m.
A rough estimation of the performance of this reference system led to the following characteristics:
Thermal resistance from absorberplate
to outside : Ru = 0.25 itr^.K/W
Thermal resistance across insulation plus
heat pipes to inside : R^p = 0.05 irr.K/W Thermal resistance in reverse direction
through insulation plus
heat pipes : Rr e v = 4 m2.K/W
Thermal resistance in air cavity, from
storage to room air : Ra c = 0.09 m2.K/W
The thermal resistance between absorberplate and heat pipe ("finfactor") is dependent on the exact detailing of the heat pipe connection to the plate. This resistance was assumed to be negligible.
T P D nummer 226.205 blad
The same goes for the resistance at the condensor end of the heat pipe
where the heat is released to the storage and/or cavity.
Effective absorption/transmission factor AT = 0.80
Ru
Heat removal factor of complete system: Fr
3 y st = ■*
mr
π —
=®·^(·\)
Ru
idem excluding storage room
: Fr
= ·«
™ — = 0.86
,-,
These rough estimations were taken into consideration for the set-up of
the optimization exercises.
These estimations have
also been used to present a preliminary
calculation of the system's performance and a comparison with an active
and a conventional passive system, see table 1 (from [3] ). As table 1
clearly shows the net heat gain from a conventional storage wall remains
far behind even under the most ideal circumstances with respect to heat
demand and inhabitants attention, as presumed for this wall in contrast
with the more pessimistic assumptions for the h.p. passive system.
For the comparison with the active system one should realize that the net
heat gain of the passive system replaces the annual heat loss through a
normal opaque wall, which means that about 50 kWh/m
2may be added to its
TPD
nummer 2 2 6 . 2 0 5 blad - 1 2TABLE 1: Preliminary calculation of the net heat gain of the high
performance passive system and a preliminary comparison with an active and a conventional passive system. Average Dutch heating season, from [ 3 ] . system HP passive system Similar active system Conventional storage wall characteristics 4 m2 collector area Fr = 0.65 4 m2 collector area Fr = 0.95 2x2 m2 area, single glass, spectral selective (ε = 0.50)
net heat gain kWh/m2
year (south orient ation, vertical)
85
105
50
remarks room with small heat demand room with small heat demand; low temperature ornaments ideal control by inhabitants; room with unlimited heat demandTPD
nummer 2 2 6 . 2 0 5 blad - 136. OPTIMIZATION OF THE DESIGN
6.1 Introduction
As described in section 4 three major components have been submitted to laboratory measurements: the heat pipes, the heat storage and the air cavity. In the following paragraphs the results of these experiments are summarised.
Detailed descriptions of the measurements can be found in (4), (5) and (6), see Appendices 1 to 3.
6.2 The heat pipes
Measurements have been performed on the thermal resistance of a few types
e
of heat pipes (figure 3 ) . These measurements are extensively reported in Appendix 1.
Copper was selected as container-wall material. For practical reasons the diameter was chosen 15 (13) mm.
Variables in these measurements were (the underlined options were or became the standard):
- heat flow : 0-150 - 270 W per heat pipe - working fluid: water/ alcohol
interior surface treatment: copper wick/none/grooved with screw thread - inclination from vertical to horizontal
length of evaporation zone : 0.10 - 0.40 m. adiabatic zone : 0.05 - 0.20 m. condensation zone: 0.10 - 0.30 m.
temperature level: 15 - 60° C (condensation zone temp.) fluid inventory: 10-20-75 per cent of evaporation zone content of uncondensable gases: none/some
The thermal resistance was obtained from the electrically supplied heat to the evaporator and the average temperature difference between evaporator and condensor. A correction was made for heat leaking through the insu^tion to and from the ambient air.
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nummer £ 2 6 . 2 0 5 blad 14 vacar In haac pip« insulation 0-50 V vacar ouc » chanaocoupleaFig. 3: Schematic view on the measurement of the thermal resistance of the heat pipes.
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nummer 2 2 6 . 2 0 5 blad 152.2
2.0
1.8
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0.5
0.4
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õmatΒr
Contents: 10 ml
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HG31 f Ι OUI [lU]4: Example fron the measurements on the thermal resistance of a heat pipe as function of heat flow; heat pipe: copper, diameter 15 (13) mm, no i n t e r n a l surface treatment; v e r t i c a l p o s i t i o n .
T P U nummer »26 205 blad_ 16
An acceleration sensor has also been used, to detect the sounds from the fluid flow (boiling).
Figure 4 shows some typical results, for an evacuated plain copper tube, without surface treatment, with 20S» of the 0.40 m. evaporation zone filled with water, in vertical position.
From the experiments the following conclusions could be drawn (see appendix 1 ) :
1. A plain copper tube without interior surface treatment gives good results in combination with water. The high efficiency in heat transfer is due to pool boiling of the fluid. The water droplets thrown upwards are effectively wetting the whole length of the evaporation zone.
Surface treatments to create by capillary action a better distribution of fluid over the heated wall surface do not result in a better performance.
If alcohol is chosen as working fluid surface treatments have an advantageous effect, but the resulting thermal resistance still remains relatively high because of the inferior properties of the fluid compared with water.
2. In moderate to high heat flow ranges the thermal resistance is 0.10-0.20 K/W per heat pipe.
If in the passive solar system heat pipes are applied with a diameter of 6 (5) mm. instead of 15 (13) mm., and 5 heat pipes are applied per m2 collector area, then the thermal resistance will be 0.05-0.10 m2
K/W, which is a good basis for the design of a high performance passive solar system. The resistance in the reverse direction will in this case be 3 m2.K/W if a 0.20 m adiabatic zone length is used through a 0.15 m
insulation layer.
For a low heat flow range the thermal resistance is 2 till 3 times higher» due to periodical intervals without boiling.
3. The thermal resistance can be decreased with some 30 per cent if the heat pipe is inclined to nearly horizontal position.
Ι π U nummer 226 205 blad _ 1 7
4. The performance is not critical with respect to the length of adiabatic and condensation zones.
5. The performance is also not critical with respect to the contents of fluid above a certain minimum filling rate. This lower limit proved to be about 40 percent of the evaporation zone, for a heat pipe in verti cal position. The upper limit is normally the fluid-height at which droplets thrown upwards from the boiling fluid reach the condensor wall. In the passive solar system, however, the heat pipes will be curved twice, when passing the insulation layer. This will prevent droplets from reaching the condensor section.
6. The heat pipe performance can be very sensitive for the contents of air or other uncondensable gases. In manufacturing careful attention should be paid to the evacuation of the heat pipe. For the same reason, it is also essential to avoid the use of materials which in combination with the working fluid can generate uncondensable gases.
Evidently the heat pipe should be leak-proof, although for safety reasons some provision is necessary to prevent the build up of dangerous over pressure in case of an incidental high temperature load.
For the detailed analysis of the system the following relation between thermal resistance and heat flow has been derived from the measurements (see also figure 5 ) :
Rhps 0.15 + 4„74/Q K/W (3)
per heat pipe with diameter 15(13) mm.
As the conclusions above and Appendix 1 show, a performance which is e.g. 50% better than relation (3) is still quite possible, depending on the details in the design.
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Fig. 5: Relation between resistance and heat transport per heat pipe (water, 15/13 mm) used as basis for the parameter study on the
T P D nummer 2 2 6 . 2 0 5 b l a d " 1 9
6.3 Heat transfer in the air cavity
A series of measurements has been performed on the heat transfer in the air cavity, with a experimental set up at the Technical University of Eindhoven (see figure 6 ) . The air cavity is created by an electrically heated hot plate on one side and a sheet of insulation material on the other. The heat transfer rate has been measured by metering the electrical supply as a function of the resulting hot plate temperature. The heat balance is checked by measuring the air flow rate at cavity outlet opening and the air flow temperatures at the inlet and outlet openings. Measurements have been performed at heights 1.80 and 0.90 m, cavity depths 0.02 to 0.10 m and heat flow ranging from 0 to 300 W/m2. Also measurements
have been performed with the opposite surface covered with aluminium foil to reduce radiant heating of this plate, in order to examine the effect of a deviating temperature difference between the two plates.
Appendix 2 contains the full report on these measurements.
The measurements have resulted in a pair of dimensionless equations (see figure 7 ) . The first equation describes the relation between total convective heat transfer and mean wall surface temperature in the cavity, the second equation describes the convective heat transfer on only the hot plate as a function of the temperature difference between this wall and the ambient air. With this pair of equations the convective heat transfer can be calculated for a wide range of cavity dimensions and conditions. The radiative heat transfer can be found with the common optical laws. The air flow rate in the cavity can be found with a third equation derived from the experiments.
The dimensionless equations with an explanation of the dimensionless numbers are given in Appendix 2.
Comparison of measured heat transfer rate with values derived from the equations show that the error remains within 10 percent.
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vertical cross section
horizontal cross section
measures ¡n m.
χ : thermocmip1 e s 7 : flow m e t e r s
Fig, 6: Experimental set-up for the measurements of the heat transfer in
the air cavitv.
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F i g . 7: R e l a t i o n d e r i v e d from measurements between the c o n v e c t i v e heat t r a r . f e r and c a v i t y c o n d i t i o n s , expressed i n the d i m e n s i o n l e s s numbers X u s s e l t and Grasshof p l u s P r a n d t l r e s p e c t i v e l y . F i g u r e 7a: c o n v e c t i v a heat t r a n s f e r f o r complete c a v i t y ; f i g u r e 7 b : the same f o r the heated w a l l o n l y .
I r U nummer 226 205 ^'*d _ 22
A sensitivity analysis using these equations showed that the heat transfer coefficient increases strongly with increased heated wall temperature, cavity depth or decreased cavity height; the coefficient strongly de creases with decreased emissivity of one of the panels. The heat transfer is much less dependent on ambient temperature or insulation thickness. Accurate information, however, can only be derived from calculations in which the interaction with the other components of the passive solar system and the interaction with the room are taken into account.
Further calculations show that the total heat transfer is 15 to 35 percent less than it would have been without a cavity construction (single wall), but the cavity makes it possible to reduce Lhe heat transfer (up to 85%) by suppressing the air flow in case there is no heat demand (see
figure 8 ) .
The heat transfer can be increased with vertical partition walls from highly conductive material (20 to 403», see figure 9 ) . A panel placed in the middle of the cavity, parallel to the heated wall has only a very small beneficial effect (10%).
The inlet- and outlet-opening areas appeared to have not a major influence on the results. This is probably due to the fact that during the measure ments the areas of these openings were at least as high as the cavity cross-section. The influence of decreased opening-areas will be investigated at FAGO in a series of measurements outside the scope of this project.
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F i g . 8: R e l a t i o n from s e n s i t i v i t y a n a l y s i s between the t o t a l heat t r a n s f e r c o e f f i c i e n t o f the c a v i t y (both i n open and closed p o s i t i o n ) and the temperature d i f f e r e n c e between heated w a l l and ambient, f o r v a r i o u s types o f opposite w a l l .
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nummer 2 2 6 . 2 0 5 blad 24 jnsrsT n 3 t « , < ε v. l^U, • * e ' / / / / / / i l r s'' /a ■■
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temperature difference between heated wall and ambient. In the cavity vertical partition walls have been provided between heated and opposite wall.
s = thickness (s = 0: no part walls), L = distance, La = conductivity.
I r i j nummer 2 2 6 2 0 5 blad_
6.4 Heat storage
Measurements have been performed on a prototype of the heat storage. The prototype has been selected on the basis of the results from preliminary calculations which showed:
phase-change (p.c.) materials with low p.c.-temperature have a high efficiency in collecting the solar energy, but it is very difficult to extract the heat from this storage to the room by natural means (ventilated cavity);
the storage material should be connected directly to the heat pipes; although this creates some extra thermal resistance before the heat reaches the cavity, this situation is much prefered over an arrangement with storage against the opposite wall, which has the disadvantage that it is now difficult for the heat to reach the storage material.
The prototype consisted of rectangular steel tubes of 1.55 χ 0.72 m2,
electrically heated at one side (see figure 10). The cavity was formed by insulation material. The cavity depth was 0.05 m.
As storage materials have been chosen: - water;
- Shell paraffine wax (Ttrans. = 52/5¿P C ) ; - Dow salt-hydrate (Ttrans. = 56° C ) .
More details can be found in Appendix 3.
The tests consisted of a period of a few hours upheating with closed cavity, followed by a period of cooling down with open cavity. For the up-heating a short, medium or long period was selected which led to three different measurements. With paraffine a extra test has been performed with a long period of upheating with open cavity.
Figure 11 shows an example of the results. This figure shows results for paraffine with a long period of upheating. The figure contains both the temperature curves and the curvee for the heat content and the change in heat content of che storage; the latter curves have been derived from the known thermal capacity versus temperature curve and the measured temperature of the storage.
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nummer 2 2 6 . 2 0 5 blad. 26 insulac ion e l e c t r i c a l l y heated hot p l a t e thermal storage' a i r ca«icy x: thermo-couplesvertical cress section
TPD
nummer 2 2 6 . 2 0 5 blad - 2 7 measured surface temperatures (K) t i m e •mxcT · NP-a-i-iev,· ■:: 1« /t/U.UjK'V*. . 2"tj:'jrt ret·. j e r c i r r e T c r . . ■¡»«.•auwiuuflrE.'*.. sarara \ total heat content (kJ) \ \imi
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/ / / / / / \ \ Ν S Ν \ measured temperatures c storage (K) i 5 u p p e i down "Χ,,^ίί^ν e l e m e r t i m e -r*uecr . f-CLSKHTi«C¿W. t » . t&U) •.■i.2*GB!.-W. <«er¡ J-H si«. .7«ri»ej ctuBaTNt «er runerrimi
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c h a n g e i n heat content (W) •scuter ι -*»-e.e«i' ι «'arsii ve*ecf< tLœtr « r «ïwary'si t i m e •^ojecr ι HftLTieNT . l*>««*TI»tI VWeCEN £L£f«NT t*T f(**yF|f«Fig. 11: Example of results from measurements on storage tubes; measurement: paraffine 52/54, long period of upheating with closed cavity.
T P D nummer 226.205 blad - 28
The thermal capacity of the paraffine has also been measured in a separate experiment on a sample of this material. Figure 12 shows the result. The same measurement on the salt-hydrate dit not succeed because of compatibility problems.
From the measurements with water the total heat transfer coefficient has been derived for comparison with the coefficient accurately measured at FAGO (see previous section) . It appeared that the heat transfer in the arrangement with the storage remains about 25% behind. This is probably due to (a) the difference in vertical temperature gradient (thermal stratification) of the cavity surfaces: the storage tests are performed with unsteady state conditions and (b) che difference in surface conditions: the storage containers had a lower emissivity.
The difference in heat transfer does however not affect the conclusions from the measurements.
From the measurements the following conclusion have been drawn:
The temperature curves for the storage clearly show the vertical gradient. Particularly the containers at the bottom remain behind. As expected, the temperature curves of paraffine and salt show at the phase-change temperatures a distinct period with significantly less change, untili during heating up all of the storage material has melted, or during cooling all of the material has solidified again Although the time scale of the figures makes it not so clear to see, temperature rise during the first period of upheating is for water less steep than for paraffine due to the difference in thermal capacities.
- Paraffine has a lower conductivity (λ » 0.1 W/mK) than water (λ « Q.óW/mK). The temperature curves show, however, that for the chosen dimensions the temperature fall over the storage remains also for paraffine rather small. The conductivity for the salt hydrate is comparable with water (λ * 0.5 W/mK).
As expected, the curves showing the momentary amount of heat supplied to or extracted from the storage are most steep in case of water, because of the 3teady decrease in temperature and thus in heat transfer.
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nummer 226 205 ^a(* - 3 0P a r a f f i n e has for a few hours a almost constant heat flux, namely when the medium remains at the phase-change temperature range during c o o l i n g . In p r i n c i p l e , t h i s i s an advantage for the phase-change m a t e r i a l .
- With the dimensions chosen, the f i g u r e s show t h a t i t takes roughly 10 hours to e x t r a c t the heat from the s t o r a g e . This may be too long. Improvement i s p o s s i b l e with use of e . g . fins or other ways to increase the heat t r a n s f e r s u r f a c e .
In a d e t a i l e d parameter study, including the i n t e r a c t i o n with the room, i t should be examined whether s a l t or paraffine are r e a l l y advantageous on a y e a rly b a s i s , compared with water as storage m a t e r i a l .
In t h i s parameter study one should remind t h a t the thermal capacity in the measurements included the s t e e l c o n t a i n e r s . In the final design of the high performance passive system the storage may be contained in d i f f e r e n t ways.
T P D nummer 226.205 blad - 31
7. CONSTRUCTIVE DESIGN AND CALCULATION OF THE CONSUMERS PRICE
7.1 Introduction
In the course of the project preliminary designs have been developed for a prototype h.p. passive solar system, applying the knowledge and experience available at the moment.
The development of the final design was performed simultaneously with the sensitivity analysis of the thermal performance. At each design stage a detailed manufacturing cost analysis resulted in a consumers' price for the designed system. A detailed report can be found in [7] , see Appendix 4.
7.2 Constructive design
From the preliminary designs it became clear that the price of the system depended for the major part upon the complexity of the detailings. The essential components only contributed with a small part to the manufac turing costs, the rest being labour costs for assembling the element from the components, and for finishing in order to obtain a product which will be accepted by the consumers.
For the final design the following major alterations have been adopted: replacing the original modular approach which appeared to be expensive to assemble;
omitting the venting possibility to the outside;
facilitate and improve the connection of the heat pipes by applying a copper plate soldered on the heat pipe ends
Three variants have been chosen: 1. with steel storage containers. 2. with aluminium storage containers. 3. without storage containers.
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Fig. 13a: Drawings of the final design of s orototype passive sola: heating system.
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7.3 Cost analysis
For the design according to figure 13 the manufacturing costs have again been analysed in detail.
The assumptions were:
A series of 400 elements
Manufacturing in a medium size factory
Costs including assemblage and mounting in the facade, including all extra neeaed provisions like window sills and other details necessary for acceptance by the consumers
Costs for manufacturing the heat pipes estimated at Dfl. 35.00 each (incl. copper plates)
Costs nave net yet been deminished with the costs for a conventional panel which is replaced by the passive system
The cost price is the amount for which these elements can be produced at this moment, on a purely commercial basis.
Table 2 shows the figures, resulting in a final consumers price of about Dfl. 1.500/m2.
Table 3 presents a breakdown of the price into the components material costs, labour coats and additional costs. This table clearly shows the minor contriDutic-n of material costs, which indicates that the design is
still toe complicated.
Confronted wich this still high consumers price, an alternative design has been made, consisting of simple sandwich modules (0,85 m χ 0,20 m) of heat pipes embedded in insulation material, absorber plate and backplate. These modules could havs a wider application. Appendix 4 shows these modules more in detail, including the manufacturing costs. Again, however, the consumers' price is high: about Dfl. 850 to 985 per or collector. Appendix 4 shows aisc that reduction by πΐ33ε-ρΓθαυ^ίοη is limited.
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nummer 2 2 6 . 2 0 5 blad_ 3 9Table 3. Cost breakdown for variant 1
percentage of: in net manufac turing costs: in consumers' price material costs
30
20
labour costs64
44
additional costs6
36
7.4 ConclusionsThe aim of this part of the research project was to develop a constructive design of a prototype high performance passive solar heating system which could be produced at reasonable costs.
First preliminary designs appeared to be too complicated and therefore much too expensive.
The cost-breakdown for the final design of the prototype showed material costs as low as expected in the first stage of the project: about Dfl. 300, for ail the materials used per m^ of collector area.
However, the net manufacturing costs still appear to become more than three times as high, because of the addition of labour costs for assembling the element and for finishing to achieve a product which will be accepted by the consumers.
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nummer 2 2 6 . 2 0 5 blad 40Table 2. Calculated consumers' price for the final design of the prototype h.p. passive solar system (Dfl. per m^)
Variant
Net manufacturing costs: including: material costs
investments in tools labour costs transport costs inspection storage Insurance Profit (59a) acquisition and advertisement (12Λ)
Gross manufacturing costs VAT (19Ä) Consumers' pi ice
1
1.053,10 + 52,65 1.105,76 + 55,29 1,161,05 + 139,32 1.300,37 + 247,07 1.547,442
1.084,64 54,23 1.138,87 56,94 1.195,81 143,50 1.339,31 254,47 1.593,773
951,95 47,60 999,54 49,98 1.049,52 125,94 1.175,47 223,34 1.398,80TPD
nummer joc. 2 0 5 blad_ 41Furthermore, additional costs (a.o. VAT) lead to a a l l - i n consumers' price again about 50% higher than the net manufacturing costs.
The result i s a prototype h.p. passive solar system with a price of about D f l . 1550/frt2 c o l l . For t h i s price the element can be delivered ready for use mounted in the facade.
A price of D f l . 350 would have resulted in a estimated pay-back period of approximately 15 years. This implies that the design i s s t i l l too complicated.
The choice of m a t e r i a l , aluminium or s t e e l , appears not to be c r i t i c a l , each having i t s (dis)advantages in the manufacturing process (variants 1 and 2 ) .
Omitting the storage, without changing the overall design characteristics, does not leao to a drastic reduction of the price (variant 3 ) .
An alternative design, constisting of single sandwich modules of absorber plate, 2 heat pipes in insulation and a back plate has a expected consumer price of about Dfl 850 per nr collector area, which obviously also is too high for use in a passive solar heating system. These modules could be
used for widely varying applications, e.g. for cooling. This would make it possible to produce larger quantities, but the cost analysis 3howed no significant series effect from which could be benefitted in that case.
The results from the sensitivity study on the thermal performance of the h.p. passive solar system will now have to make clear whether it is possible to use other design principles which would make the system cheaper.
The details which need attention from the constructor's point of view are: the connection of heat pipes to absorber plate;
the connection of heat pipes to storage; the number of heat pipes;
the storage material;
the storage container, shape and material; the venting openings.
Other options to decrease the cost-benefit ratio are a system without storage or the use of an electric fan to increase the performance. In the sensitivity study on the thermal performance these options will be taken into consideration.
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226.205 blad- 4 38. Heat gain calculations of the h.p. passive solar heating system
8.1 Introduction
Simultaneously with the cost analyses an extensive parameter study has been performed with an unsteady state model of the high performance passive solar system, under realistic climatic conditions and with realistic hourly heat demand, indoor air and wall temperatures for a few types of dwellings and occupants' behaviour, for typical Dutch circumstances:
The characteristics of the h.p. passive element in the reference case have been derived from the previous investigations concerning the major components of the system (see table 4 ) .
The NEN 5060, Short Reference Year for weather conditions f 8] has been used for the climatic data.
The hourly heat demand and indoor conditions have been calculated for a typical dutch house in a row as built during the last decade.
It is assumed that only the ground floor is heated: Minimum temperatures: ground floor : 8 - 24 h: 21 ° C
0 - 8 h: 16° C sleeping rooms: 0 - 24 h: 10° C Maximum temperature : 26° C.
Internai heat: hourly pattern, mean value: approx. 400 W. Insulation and ventilation (incl. infiltration):
Moderately insulated dwelling with exchange rate 1.6 h.~^ Heavily insulated dwelling with exchange rate 1.0 h.~ .
In [9] the calculations have been reported more in detail, see Appendix 5, In publication [10] results are presented- from an analysis which focussed on the optimal type and size of the storage.
8.2 Calculations
A number of parameters have been subject to variations in order to find the sensitivity of the results for individual design changes. This parameter study forms tne basis for a optimized h.p. solar heating system.
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nummer 2 2 6 . 2 0 5 blad - 4 4Table 4: Reference values for the properties of the h.p. passive element as basis for the parameter study.
Panel width Panel height Heat pipe number Heat pipe length
Heat pipe ext. diameter Heat pipe int. diameter Heat pipe conduct. Cover layers
Cover transmission Cover emiss.
Absorber abs. fet. Absorber emiss. Absorber conduct. Absorber thickness Weld mean thickness Weld width
Weld length Weld conduct.
Weld effect distance Insulation thickness Insulation conduct. Storage cylinder width Storage cylinder height Storage cylinder thickness Storage cylinder spec, mass Storage cylinder spec, heat Storage cylinder mass Storage material Storage mass
Cavity emiss. ext. Cavity emiss. int. Cavity depth
Cavity wail thickness Cavity wall conduct. Cavity control
Correction factor HP resist. Correction factor cavity resist,
2.00 1.00 10 0.20 0.0080 0.0060 350.00 1 0.85 0.95 0.95 0.10 210.00 0.0025 0.003 0.010 0.200 350.00 1.00 0.150 0.030 m m _ m m m = ' w7mK] _ -_ -'W/mKl
