Active Heating

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Education of Architects in Solar Energy and Environment page 1

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Active solar air heating systems

Active solar air heating systems differ from passive solutions because solar gains arecontrolled and distributed using a mechanical system (a fan). The efficiency can be higher, accordingto the design of the system. But the cost is often also higher and such systems must be carefullymaintained. This is why architectural integration and directions of use will be particularly consideredin this section. A simpler solution, but leading to a lower solar fraction, consists in preheating theventilation air. Air renewal contributes between 20% and 40% in the total heat losses of a standardhouse in mid Europe. In low-energy buildings this amount rises to up to 70% of the total heatingenergy demand. Thus, with air preheating the total energy demand of such houses can easily beenreduced to less than 40 kWh/m2 and year.

Air heating systems can be classified in two main groups according to the air circulationpattern. In closed loop systems, air is circulated from the house to the collector and flows back to thehouse. In open loop systems, air is taken from the outside, flows through the collector, is thencirculated into the building and flows back to the outside. This second configuration corresponds topreheating of ventilation air.

CLOSED LOOP

HEATING

OPEN LOOP

7

PREHEATING OF VENTILATION AIR

AIR COLLECTOR 1 : TRANSPARENT COVER, 2 : ABSORBING SURFACE, 3 : INSULATIONDISTRIBUTION AND STORAGE 4 : FAN, 5 : THERMAL INERTIA

VENTILATION, AIR RENEWAL 6 : EXTRACTION FROM HUMID ROOMS, 7 : FRESH AIR INLET

The collector is formed of a transparent cover which transmits solar radiation while reducingheat losses and an absorber, i.e. a dark surface where solar radiation is transformed into heat and


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transmitted to the air. The collector should be insulated at its rear side in order to protect the buildingfrom overheating in summer, and to avoid heat losses in winter at night and during cloudy days (thecollector can be very cold during these periods).

Air is circulated in the collector using a fan. This fan is controlled in closed loop systems : itfunctions only if the temperature is higher in the collector than in the building. In open loop systems,the fan is always on because ventilation of the building is always needed. It extracts air from the humidrooms of the building (e.g. bathroom, kitchen), and fresh air is drawn from the outside into the otherrooms (e.g. living room, bedrooms). Natural ventilation could be an alternative in climates and siteswhere it is possible. Thermal inertia can be part of the building envelope (slabs, walls) in order to storeenergy and to reduce temperature swings, thus improving thermal comfort.

1 Closed loop systems

1.1 Principle

The solar collector can either be integrated in the roof (case A, cf next figure), or constituted bya sunspace (case C). We may also have both types in a single building (case D). A supplementarystorage can be formed of two slabs between which circulates the air heated in the collector (case B).This storage could also be larger, filled with rocks and situated under the building or in its center. Butsuch storages are not proven economical.

A) Roof collectors1

The two possibilities are either to buy industrialised collectors or to build a collector on siteusing a transparent cover (1) and an absorber plate (2) made of a metal sheet, aluminium or steel,painted black and insulated by e.g. 10 to 20 cm of glasswool (3) at the rear side.

The fan (4) is controlled by a differential thermostat (see 1.2 for more details), which sets thepower on if the temperature in the collector is higher than the temperature in the main room (e.g. livingroom) plus a differential (e.g. 5 K). Another thermostat is used to protect the building againstoverheating : it stops the fan if the temperature in a room is higher than a maximum temperature fixedby the occupants (e.g. 25C). Rather thick slabs (5) may be integrated in the building structure inorder to store energy and reduce temperature swings.

In summer, it is essential to stop the system (the fan is switched off) and to cool the collectorsby circulating air : openings at the bottom and the top of the collector provide sufficient air circulation.

B) Energy storage

If the building is occupied also at night, it could be interesting to store the energy collectedduring the day by circulating air between slabs (case B in figure hereunder). This would provide theheated rooms with a more constant heat flux.

The heat transfer between air and a slab is rather low : solar radiation transmitted throughglazings and hitting directly the slab is more efficiently stored than energy transmitted by convectivetransfer with air. A supplementary energy storage can be achieved by a rock bed storage, because thearea of the rocks and thus the thermal contact with air is much higher. But such storages are rarelyeconomical : it is either necessary to excavate the ground under the building, or to reduce the livingspace if the storage is included in the building itself (e.g. vertical storage in the center of the building).

1 Patent TROMBE-MICHEL, 1975. This patent being more than 20 years old, the invention is now in publicdomain.


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A) ROOF AIR COLLECTORS B) ROOF AIR COLLECTORS + GROUND STORAG

C) SUNSPACE D) ROOF AIR COLLECTORS + SUNSPACE

C) Sunspace

A sunspace can also be an efficient solar collector, particularly a rather narrow one covering alarge part of a south facade. But a sunspace should also be a comfortable space for occupants andpossibly plants. In such cases, thermal comfort should be carefully checked (cf section on susnpacesand atria) and solar protection should be planned (opaque roof or shading device, ventilation in

summer). If necessary, one should check that the temperature is always above freezing in thesunspace. An appropriate transparent cover should be selected accordingly.It is advised to reduce temperature swings in the sunspace using a masonry wall (5) and a

rather thick slab. Thermal insulation (3) should thus be placed on the internal side of this wall. Theabsorbing surface (2) should be painted rather dark in order to improve efficiency. The fan (4)providing air circulation should be controlled the same way as in case A).


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D) Roof collectors and sunspace

Combining both components could increase the solar fraction of the building (fraction of theheating load provided by solar energy). In this case, it is advised to circulate air before in the sunspaceand then in the collector, because heat losses are in general lower in flat plate collectors.

But using the sunspace for preheating of ventilation air (see open loops) is often moreefficient.

1.2 Selection and sizing of components

- transparent cover

The choice of the transparent cover depends on the climate, because the set point temperature inthe collector is always around 19C plus a differential (e.g. 5 K). Energy is not stored in the collector,unlike in solar walls. High insulating cover is thus rarely necessary. But a U value of less than 2W/m2/K is recommended in mid European climates. This corresponds to a low emissivity doubleglazing, or a three panes polycarbonate plate (or 5 cm transparent insulation). The following tablepresents productivities using various transparent covers (simulation results).

Type of transparent cover U (W/m2/K) productivity (kWh/m2/year)low emissivity double glazing orthree panes polycarbonate plate

2 140

5 cm transparent insulation 1.5 180

10 cm transparent insulation 0.9 200

In mediterranean climates, roof collectors are unsuitable because of overheating risks : thematerials could be dammaged if the collector is not sufficiently cooled. A south oriented sunspace canbe used, but an opaque roof is advised. Openings should be placed at the bottom and at the top of thesunspace in order to ventilate it by thermosiphon. In such climates, the transparent cover can be simplymade of a single glazing (possibly a double glazing in mountainous regions).

- absorber

The absorber should be painted black or dark. If the transparent cover does not include a lowemissivity coating, painting the absorber with a selective coating could increase the performance. A

corrugated metal sheet leads to a higher convection coefficient compared to a flat surface, because theair flow is more turbulent. The layer of insulation at the rear side of the collector should be thickenough to protect the building from overheating in summer. In practice, 16 cm of glass wool isappropriate.

- sizing the system

Sizing the system is more related to economical and aesthetical aspects than to an "energyoptimisation". The area of collector should be large enough to justify the investment of the controlledfan and air ducts necessary to control the whole system. On the other hand, the productivity decreaseswith the area of collector. Too large collector arrays would thus not be economical. In practice, acollector area of 15 to 25% of the heated area is adapted.

- thermal mass

It is difficult to use thermal mass with air heating systems, because the heat transfer between airand e.g. a slab or a wall is low. Rock bed storages allow to increase the contact area, but theirarchitectural integration is uneasy. "Architectural" thermal mass (slabs and walls) hardly increase the


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performance of air heating systems, but do improve thermal comfort. They absorb direct solarradiation transmitted by glazings and reduce temperature swing.

The following graph gives example simulation results for a mid European climate (north-eastof France) and a single family house equipped with solar collectors on the roof. Three levels of inertiaare compared, considering an example temperature profile in summer. The low inertia configurationcorresponds to a wooden frame structure without any masonry. A 16 cm thick concrete slab and lightwalls constitute the "medium inertia" case. If the walls are also made of 16 cm concrete like the slab,high inertia is obtained.

We can see the influence of inertia on thermal comfort. Thermal comfort can be furtherimproved by night ventilation and using shading devices on windows.

- air circulation

As mentioned above, sunspaces can also be used as solar collectors. But their performance isstrongly related to the occupants behaviour. Particularly, the door between the sunspace and theadjacent room should rarely be open. The following table presents heating demands and energysavings for various possible types of behaviour and system configuration :

- the occupants open the door very rarely, and air exchange is obtained using a controlled fan (optimalcase) ;- the door is open 50% of time at random (random case) ;- the occupants open the door at night and during cloudy days, and close the door during sunny days(worst case) ;- the occupants open the door very rarely, and the air circulation is in open loop (preheating ofventilation air).

These figures correspond to a specific building (100 m2 single family house and 10 m2sunspace) in the climate of Paris.

Sensitivity study on air circulation pattern between the building and an attached sunspace

Air circulation pattern Heating demand(kWh/year) Energy saving (%)

Reference building (without sunspace) 10,700 -optimal air exchange 10,200 +5%random opening of the door 11,100 -4%worst case 11,800 -10%

open loop (preheating of ventilation air) 9,500 +11

Example summer profile

hours

2 2

2 3

2 4

2 5

2 6

2 7

2 8

2 9 low

inertia

medium

inertia

high

inertia

high

inertia

+ventilation

+shading device


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We can see that a sunspace is not a very efficient solar collector in a closed loop configurationand in the climate considered here (Paris). But according to experiments and other calculations, it canbe very efficient in mediterranean climates. In rather cold climates, it should be carefully designed, i.e.the door should be closed automatically and a controlled fan should circulate air when appropriate.Otherwise the sunspace increases energy consumption (cf table above, random and worst cases). In 2we shall see that sunspaces perform better in open loop (preheating of ventilation air).

Concerning the distribution of solar heat in active systems, the air flow rate should not be toohigh in order to avoid air draught, noise and dust movement. A rate of 1.5 air change per hour (ach) issufficient (1 ach means that one volume of air included in the building is circulated per hour). This

corresponds to a 100 W fan for 250 m3 of building (20 m2 of collector).In order to reduce the energy consumption of the fan, natural thermosiphon movement shouldbe respected. Warm air should thus be blown in rooms at a high level, and cold air should be drawnfrom the ground level of the building before flowing to the bottom of the collector. Air is heated in thecollector and raises naturally. The fan draws air from the collector which is thus at a lower pressure.Because on site built collectors cannot be completely airtight, air flows from the loft into the collector.This brings a supplementary amount of ventilation air, which is preheated. Other fresh air input intothe dwelling could even be lowered in compensation, in order to save energy whithout impairing airquality.

The air ducts and the fan should be carefully insulated to reduce distribution losses and toavoid condensation : hot air flowing from the collector is cooled in these components andcondensation might occur if the ducts and fan are too cold.

A grid situated at the cold air inlet avoids dust and insects to be drawn into the collector.

- controlThe control system should :

maximize energy saving by giving priority to solar energy versus conventional back-up ; ensure thermal comfort by stopping the system in case of overheating.

The first requirement is met thanks to a differential control. One sensor is located in the mainroom (e.g. living room), preferably near the thermostat (if it exists) of the back up heating. We noteroom the corresponding measured temperature. The other sensor is situated in the collector at amedium level and measures a temperature coll. The differential control starts the system as soon as :

coll. > room + where is the differential, and stops air circulation if :

coll. < room

In order to avoid constant switching on and off of the fan,

can be fixed e.g. to 5 K. A too high valueof would reduce solar gains because the system would hardly function in sunny winter days. If a toolow value is chosen, the fan would be started immediately at the first morning sunlight. As soon as airwould circulate, the temperature in the collector would drop and the system would be switched offagain. The cycle would be repeated a lot of times, accelerating the wear of mechanical components.

Such a differential control gives priority to solar energy : even when a back up heating isneeded (cold winter days), solar energy can be gained as soon as the net energy balance in thecollector is positive. The back up just complements the solar system. Air flows into the collector atroom (fixed by the occupants, e.g. 19C). The collector temperature is thus minimal which improvesits efficiency by reducing heat losses.

At night or during cloudy days, the collector is cold and reverse thermosiphon may occur : theair, cooled in the collector, is heavier and flows down, cools the dwelling and, becoming warmer, flowsback to the top of the collector. In order to prevent from this phenomenon, an air flap can be mounted

at the cold air inlet (situated in a bottom room, e.g. the entrance hall) and equipped with a motor (suchdevices are common in air conditioning systems). This flap should be closed by the differential controlafter the fan is closed and opened before the fan is on. A contactor should be used to ensure that thefan can function only if this flap is open. This prevents from damaging the fan.

Concerning thermal comfort and prevention from overheating, a thermostat control can beused. Its sensor should be located in a room where the risk of overheating is the largest (usually a


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room situated under the collector, because the south facing roof is the hottest part of the building inmid season). The thermostat control stops the fan if the temperature in this "hot room" is higher than alimit max fixed by occupants (e.g. 25C). max should be higher than the thermostat set point of theback up heating plus at least 3K, so that priority is always given to solar heating. In summer, the wholesystem is of course stopped.

1.3 Architectural integration

In the example considered, a collector has been integrated in the roof (cf next figure), replacingthe traditional cover by a translucent insulation component. The resulting collector space is situated in

front of a bedroom (cf next plan). This configuration has been adopted because it is economical :industrialized air collectors hardly exist on the market and they are expensive.A central air duct, thermally and acoustically insulated, contains the fan sucking warm air from

the collector. It is connected to a network that distributes warm air into several rooms of the dwelling.Cold air is sucked at the ground level, passes through a filter and flows back into the collector.


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The next drawing shows the link between the upper roof connection and the glass roofconstituted by transparent insulation panels, mounted on an aluminium structure. The roof has beenvery carefully designed in order to ensure air and water tightness. The internal wall of the collectorspace is made of special panels (aglopan) that resist at high temperatures without emitting harmfulgases. The black mat painting covering the absorber has also been selected to avoid air pollution. Allcomponents must of course respect the ten years guarantee needed in the construction sector. Specificregulation may apply to air systems also at a national level.


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The details of the structure are presented hereunder. Aluminium profiles constitute a frameupon which transparent insulation panels are mounted using a sealing joint. The capillary material isprotected by two quench glass panes, and ventilated to reduce condensation. Thermal bridges in thestructure could be reduced by injecting insulation foam in the profiles.


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.4 Maintenance and Directions for use

Once installed, the active system should function automatically because it is switched by boththe differential and thermostat controls. Occupants do not need to intervene all year long. At the end ofthe heating season, the electric supply of the active system (fan, motor of the flap) should be off andthe collector should be ventilated by opening louvres or small windows. At the beginning of theheating season, the collector should be closed and the fan should be supplied again with current("winter position").

Besides this simple operation, a maintenance is advised each year (like for a standard boiler) inorder to check the system. The most convenient is to do the following test when setting the system in

its winter position. The objective is to check that the fan functions if the temperature in the collector ishigher than in the heated room. It is preferable to start the system during a sunny day but if theweather is cloudy, it is possible to cool the room sensor using cold water so that the collectortemperature is higher than the room temperature plus the differential. The flap should be opened bythe motor and the fan should function after one or two minutes. If the temperature in the hot room ishigher than the comfort limit fixed by occupants, just increase this limit for the test so that thethermostat allows the fan to function.

It is also important to clean the grid at the cold air inlet if necessary.

1.5 Example realisation

- Active houses of Aurore solar estate, Mouzon (north-east of France) - Architect Jacques Michel

Description of the active systems (see general description of the project in the solar walls section)


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In the active system, a transparent cover forms a solar air collector on the roof (next Fig.), eitherwith a 5 cm OKALUX capillary structure or with 16 mm of triple wall polycarbonate plates producedby CELAIR. The warm air is inducted into the dwelling by a controlled fan. The air circulation is alsoshown in next Fig. The flow rate can be varied between 1 and 2.5 ACH. In practice, and after

predictive calculations, a flow rate of 1.5 ACH, corresponding to 480 m3/h was adopted. A highervalue does not significantly increase the performance, but leads to draught and noise. The controlsystem allows the air to flow if the collector temperature is higher than the dwelling temperature plus adifferential (which can be set between 0 and 20 K). A thermostat switches the fan off if the insidetemperature becomes too high.

Active system, air collector

In summer, the fan is off and the collector is ventilated by two openings. The opaque insulation(16 cm rock wool) protects the rooms underneath the collector against overheating.

The supplementary investment cost is given below for two types of transparent cover.

Cost for each system in Euro 1992

configuration(2 x 50 m2living area)

solar overcost total cost cost per m2

of living areaActive system, polycarbonate 9,000 76,000 760

Active system, 5 cm OKALUX 16,000 83,000 830

2.2 First predictive simulation results

The simulation tool COMFIE (cf. design tools section) has been used during the design inorder to compare various possibilities :- transparent cover (single or double glazing, polycarbonate plate, 5 or 10 cm capillaries);


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- control of the active system, air flow-rate.Simulations were performed over a heating season. The climate considered is the Short

Reference Year (Lund, 1985) of Nancy (SRY) : 8 typical weeks, 2 per season. It is assumed that thereis no air leakage in the air collector, and that the control system functions correctly. The firstmeasurements showed that in fact the air collector is not airtight, though the construction was carefullydone. This effect corresponds to a preheating of ventilation air.

According to these predictive simulation results, the annual heating consumption is reducedand the solar fraction can reach 40 to 45% according to the system (cf table below). This avoids toreject 2 tons of CO2per year and house (electric heating with fuel or coal power plant during the peakhours).

Comparative simulation results for the different systems

configuration(2 x 50 m2living area)

heating demand(kWh/a)

demand per m2

(kWh/m2/a )solarfraction

Active system, polycarbonate 6,000 50 40

Active system, 5 cm OKALUX 5,300 44 44

Summer Results

Attention was paid to the highest absorber temperature in the collectors to be sure that the limitadmissible for the polycarbonate (130C) will not be reached. The measurements showed the highest

absorber temperature to be 105C, and the polycarbonate temperature is lower. The polycarbonatematerials are guaranteed by manufacturers for 10 years under propper conditions. Previous projectsusing such materials as transparent cover have shown a satisfying durability, and the quality ofproducts is improving, particularly concerning U.V. protection.

Autumn Results

We studied in detail the period from October 25th to November 1st 1992 (Fig. below). First,there were three cloudy days, followed by 4 sunny days and one overcast day at the end. Therefore,this period is a good representation of the possible weather situations during mid-season. The ambientoutside temperatures at the begining of the period ranged from 5C (night minimum) to 12C (daymaximum). They then dropped to -2C and +3C during the last day.

Autumn climate conditions in Mouzon, 25/10/92 - 01/11/92


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Absorber temperature in the collectors of the active system, 25/10/92 - 01/11/92

The active system constitutes a real heating system when the weather is good. Then, the absorbertemperatures can reach 43C for the polycarbonate plate (PC) and even 52C with the transparentinsulation (TI, cf Goetzberger and Jesh) component. The system does not work during bad weather,because the temperatures are not higher than 25C. They drop at night to 3C, even dropping tofreezing the last two nights. The inside temperatures of the houses oscillate between 15C and 25C,

depending on the solar irradiation and on the heating thermostat set point, which is chosen by theinhabitants. In the house studied, the thermostat was lowered at night.

Results during the coldest period

We analysed the week from December 31th to January 7th 1993. The first four days of thisperiod were sunny and the temperature dropped to -12C. The next two days were unsettled and thelast two were very cloudy but with increased temperatures (between 5C and 7C).

The absorber temperature of the active system rose to 30C - 35C during the first 4 days (whenthe fan was on), to 25C during the fifth day, to 20C at the sixth and then it stayed at about 10Cduring the two cloudy days (the fan remained off). The room temperatures varied, depending on thecontrol chosen by the inhabitants. The fan was on between 12 o'clock and 18 o'clock during the sunnydays. Even during this cold period, the energy used for heating purpose was greatly reduced. On the

contrary, the fan did not work during the cloudy days, because the air temperature was too low in thecollectors.

Comparison between measurements and predictions

The heating demands, measured by electricity meters, have been integrated over a whole heatingseason. They are compared in table below with the predictions obtained by simulation.


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Measured and predicted energy consumption on a heating season

house measured consumption(kWh)

predicted consumption(19C)

actual meantemperature

actual degreedays

3 (passive, TIM) 3,560 (1) 7,000 16.32 21873 bis (passive, PC) 8,218 7,700 18.97 27414 (active, PC) 3,050 (1) 6,000 18.51 (2) (2)5 (active, TIM) 4,685 5,300 18.64 (2) (2)7 (passive, TIM) 7,037 7,000 18.75 27199 (passive, PC) 5,051 7,700 16.74 2296

(1) The mechanical ventilation was stopped in this house, it would add a 3,500 kWh heating demand.(2) 6 weeks are missing due to technical problems, this does not concern the electricity meters.

The discrepancies between measures and predictions are mainly caused by the occupantsbehaviour. First the simulations were performed assuming a constant 19C thermostat set point and aSRY (Short Reference Year, see 2.2) for Nancy (2773 degree days). The actual climate was slightlydifferent (2745 degree days). In the reality, some people stop the heating while they are workingduring the day and lower the set point at night and in the bedrooms. This is shown by the actualdegree hours column. Some occupants have stopped the mechanical ventilation, which would haveprovided a constant 0.6 ach air renewal. The corresponding load is 3,500 kWh per year (for a constant19C inside temperature). The internal gains for lighting etc. may also explain certain discrepancies.Finally, the physical assumptions of the model and the uncertainty on parameters producesupplementary errors.

In order to compare the different systems, it is necessary to eliminate the effect of different usersbehaviour. Simulation allows to perform such an analysis, provided that some parameters (e.g. thermalbridges) are identified or corrected. The methodology adopted is the following (like in the solar wallssection). We decided to study in detail the coldest week, during which it is assumed that the occupantsdo not open windows or doors for a long time, air being renewed by the mechanical system. Thermalbridges through collector frames were identified during the cloudy period of this week, when solargains are negligible. The product of the transmission factors of transparent covers by the absorptionfactor of the absorber was slightly modified in order to meet the measured temperature profile duringsunny days. There remains a temperature difference during clear nights due to insufficient modellingof radiation towards the sky.

This parameter identification was then checked during another week, in mid-season. Theagreement was rather good, except for one day during which we suppose that condensation occuredon the inner face of the outer pane of the transparent cover.

Using this corrected model, we simulated all systems using the same occupancy pattern as in thepredictive calculation : a constant 19C set point temperature, a constant 0.6 ach air renewal andconstant 400 W internal gains. The demand loads obtained on a typical year (SRY) were quite similarto the predicted values. Measured heating consumption was also corrected in terms of the measureddegree days, assuming a linear dependence. Also, the load corresponding to mechanical ventilationwas added in the houses where occupants had stopped this system. Comparative results are presentedin fig. below. According to the social housing company, the mean heating demand in the region forsuch detached houses is typically 10 to 11,000 kWh.

Comparison between measured and calculated heating consumption after correction


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Concerning active systems, the electricity consumption of the 100 W fan, functionning onlyduring sunny hours of the heating season, is less than 200 kWh.

Both calculations and measurements give an advantage to the active systems (houses 4 and 5),for which the solar fraction reaches 40%. But these systems need a maintenance twice a year. In may,the collectors are to be naturally ventilated from outside and the inside air circulation must be stopped.In october, the system must be set in winter position, and the control system must be checked.

References

[1] A. Goetzberger, J. Schmid and V. Wittwer, Transparent insulation system for passive solarutilization in buildings, 1st E.C. Conference on solar heating, Amsterdam, 1984.[2] A. Goetzberger, Special issue on transparent insulation, Solar Energy vol. 49 number 5, 1992.[3] H. Lund, Short Reference Years and Test Reference Years for EEC countries, EEC contractESF-029-DK, 1985.[4] L. Jesh, TI1...TI7, International meetings on Transparent Insulation, Birmingham, Freiburg,Delft, 1986...1995.[5] J. Michel, Patent ANVAR TROMBE MICHEL BF 7532921 (France 28/10/1975), AI 7532921(France 28/10/1975) and addition: Patent "Stockage thermique" MICHEL DIAMANT DURAFOUR75-106-13, Paris, 1975.[6] B. Peuportier and I. Blanc Sommereux,Simulation tool with its expert interface for the thermaldesign of multizone buildings, Int. Journal of Solar Energy, 8/1990 (received in 1988).[7] S. Soler, M. Gery and J.L. Chevalier, Theoretical and experimental study of the behaviour of

multi-wall ribbed materials under solar radiation, Transparent Insulation Workshop TI 6,Birmingham, 1993.


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2 Open loop systems

Building are increasingly insulated, so that their ventilation contributes more and more to theglobal heating load. For a standard single family house in a mid-European climate, ventilationconstitutes around 20% to 40% of the heating load. This proportion may reach 70% in low-energybuildings. Preheating of ventilation air is the most efficient application of solar collectors, because itcorresponds to the minimum collector temperature and thus the minimum heat loss.

A) FACADE AIR COLLECTORSC) SUNSPACE

B) ROOF AIR COLLECTORSD) GROUND PREHEATING/COOLING


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Collectors can be integrated in a facade or a roof. They can be flat plate or constituted by asunspace. Underground air ducts may be used in winter to preheat ventilation air, and in summer tocool it. An alternative to solar or ground preheating is heat recovery through a heat exchanger : freshair is heated in contact with extracted air, drawn by a mechanical system (fan). Such equipment hascommonly an efficiency of 0.5. Higher theoritical values are difficult to reach in practice over a longterm, because exchangers are covered with dust. The 0.5 figure is what is achieved using standarddevices. More complicated heat exchangers should be compared to active solar. According to theclimate and the use of the building, it could be more appropriate.

2.1 Principle

a) facade air collectors

Configuration A1Configuration A2

Configuration A3

Like in a closed loop, solar radiation, transmitted through the transparent cover (1), is absorbedon the black or dark surface (2) of the wall and transformed into heat. This heat is transmitted to thefresh air drawn by a mechanical system (fan) or a natural ventilation which extracts air from humidrooms (kitchen, bathroom). Unlike in a closed loop, air is drawn from the outside and may depositdust on the transparent cover (configuration A1). This is why the absorber (2) should be attached tothis cover (configuration A2). The air flows then in the air gap (4) between the absorber and the

internal opaque wall without any contact with the transparent cover. In configuration A3, the absorberis in contact with air on both sides, and the use of corrugated iron enhances heat transfer. But air isstill in contact with the transparent cover, and could make it dirty. In such cases, the transparent coverneeds to be opened for cleaning.

In all cases, the opaque wall should be insulated (3) because the air gap (4) is cold in winter,and hot in summer. Heat transfer between air and masonry is very small, so that thermal inertia is


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hardly useful in configuration A2. It may be useful in configuration A1 because solar radiation hitsdirectly the masonry. But the corresponding increase of performance depends on the climate and theuse of the building (day/night occupancy).

Solar walls should be facing south (or any orientation from south-east to south west) tomaximize solar gains and minimize overheating risks. They should give fresh air to non humid rooms(e.g. living room, bedrooms) : air should be rather extracted from humid rooms.

The louvres should be switchable from winter to summer position, when preheating ofventilation air is not needed anymore. Two possibilities exist : fresh air can be either taken directlyfrom the outide (like in winter, configuration "T"), or from the building (configuration "I"). It cools thecollector and flows out by thermosiphon. If air is drawn from the building, fresh air is taken from the

north side to cool the building (cf next figure, configuration "I").

Configuration "T"Configuration "I"

In configuration A2, the absorber is located near the transparent cover. It is thus important tocheck that the temperature of this cover does not impair the durability of the materials used(particularly using plastics). Thermal inertia (5) also contributes to improve the thermal comfort level.

b) roof air collectors

The principle is the same as for walls, usually without thermal mass. If configuration A1 ischosen, the transparent cover should be accessible for regular cleaning. Configuration A2 is riskybecause it could lead to very high temperatures at the absorber in summer. In both cases, the collector

should be very well insulated (3) from the dwelling because solar radiation is in summer higher thanon vertical walls. Roof collectors have a better efficiency than solar walls, but air is heated at a highlevel, which is against natural thermosiphon circulation and could increase the electricity consumptionof the fan which extracts air from humid rooms.

c) sunspaces

A sunspace can be equipped with louvres : fresh air flows from outside at the bottom (7), isheated and flows to the dwelling from which it is extracted either by a fan or natural circulation (6).Thermal inertia (5) may increase both efficiency and thermal comfort in the sunspace. The wallconnecting the sunspace and the dwelling should be insulated (3) on its inner side. Due to theincoming fresh air the temperature in the sunspace is rather cold. This is why the wall should beinsulated, and its possible glazings should have a low heat loss coefficient (at least double glazing is

recommended). The door should be closed as often as possible (a device could be used to close itautomatically). On the other hand, the envelope of the sunspace does not need such level of thermalinsulation, e.g. single glazing is enough.

In summer, an opening situated at the top of the sunspace provides natural circulation andreduces overheating. Fresh air could be introduced into the dwelling rather from the north side, e.g. byopening windows, and drawn into the sunspace by chimney effect.


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d) underground pipes

Due to thermal inertia, underground temperature is swinging much less than outsidetemperature. At a certain depth (depending on ground composition and humidity), it can even beconsidered as constant. Underground pipes can thus be used to preheat ventilation air in winter, orcool it in summer. This system can be used in combination with the previous ones. Usually, a depth of1-2 meter is adopted.

2.2 Selection and sizing of components

Solar preheating of ventilation air should be compared with heat recovery using a heatexchanger. For buildings where the area of collector is limited (because the south facades are small orequipped with large windows) and in which the ventilation flow rate is high (e.g. schools, officeswhere the number of occupants is high), heat recovery is often more efficient. But in other cases, theproductivity of solar collectors used for preheating of ventilation air is quite high and this applicationis certainly the most efficient way to use a sunspace as an energy saving component.

As mentioned above, the temperature of the collector is low because air flows directly from theoutside. Heat losses are thus reduced and advanced glazing is not needed in such systems : a heat loss

coefficient (U-value) of 3 W/m2/K is enough (e.g. double glazing or two panes polycarbonate plate).The absorbing surface should be black or dark, and heat transfer with the air can be enhanced usinge.g. corrugated iron. The insulation layer between the air gap and the dwelling should reduce the heat

losses in winter, and protect from overheating in summer. An overhang or vegetation could alsocontribute in reducing solar gains during hot months.

As for a closed loop, the solar fraction increases but the productivity decreases with thecollector area.

Heat transfer being low between air and masonry, thermal inertia should rather be provided inthe dwelling by slabs than in the collector itself. Such components absorb direct gains transmitted bywindows and reduce temperature swings. On the other hand, the connecting wall of a sunspace shouldbe heavy and store energy, which is useful if the building is occupied also at night.

Concerning heat distribution, the ventilation air is securely drawn into the building by amechanical system, which extracts air from humid rooms (kitchen, bathroom) and creates a pressuredrop. Natural circulation could be an alternative in certain climates, where the wind direction and speedare not varying too much. The fresh air inlet can be located at the bottom of the collector or in theground. In this case, ventilation air can be cooled in summer. The louvres between the collector and thedwelling must be switchable to stop the system in summer.

2.3 Architectural integration

The following figure shows a system combining a sunspace and an air collector. Thisconfiguration increases solar gains, and also natural ventilation by chimney effect, used in summer forcooling purposes. An opening at the bottom of the sunspace is manually operated by the occupant. Itallows fresh air to flow into the sunspace, where it is preheated before flowing into the dwelling andensuring air renewal.

Very thick masonry walls (46 cm plus 8 cm building finish) provide a high thermal inertia and

transmit solar radiation from the sunspace into the dwelling with a time lag. Thinner walls would besufficient, but clay has been used here for the rough masonry. This clay has been obtained from thebuilding site itself when excavating the cellar.


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2.4 Directions for use and maintenance

Preheating of ventilation air is of course needed only during the heating season. The systemshould thus be disconnected in summer, usually by switching louvres (through which air flows fromthe collector into the dwelling) appropriately. Another outlet should thus be provided at the top of thecollector. If possible, a north facing window should be used as inlet (configuration "I") rather than thecollector inlet (configuration "T"). Air should circulate freely within the dwelling, e.g. a space shouldbe left under doors (usually 1 or 2 cm high).

Concerning sunspaces, the door connecting the sunspace and the dwelling should be closed asoften as possible during the heating season. This should be explained to occupants, and a device could

even be used to close automatically the door. An opening located on the top of the sunspace shouldallow hot air to flow out in summer.The maintenance of such systems consists in cleaning the transparent cover when needed if the

air circulates in contact with it (configurations A1 and A3).

2.5 Example realisation

SOLAR WINE GROWER HOUSE AT MAILLY CHMPAGNE - 1985Architect Jacques Michel

According to a traditional clay and wood architecture, the house is built using clayblocks (32 cm x 16 cm x 11 cm) providing a high thermal inertia. A timber frame ensures globalbrace. The attached sunspace is continued with an air collector integrated in the roof. The sunspace iscooled in summer by natural ventilation through two upper openings. A supplementary opening in the

collector provides a chimney effect that sucks air from the dwelling. Fresh air is taken from an inletsituated in the north facade, where the temperature is a little cooler. This system has been functioningduring the last ten years. The house is both energy efficient and very comfortable.

The following plan shows the organization of the various parts of this wine grower'shouse. A cellar, built using concrete, contains the barrels. The upper slab constitutes a platform. Asupplementary refectory is joined for the vintage season. The building is compact, its south facade


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being highly glazed whereas the north facade is protected against wind and the west facade againstrain.

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