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ACTIVE SOLAR DESIGN WITH THERMAL STORAGE
ANDREW DUNCAN
Massey Centre for Energy Research, www.energy.massey.ac.nz
Email: andy@eqo.org.nz
Abstract
In New Zealand, passive solar design has significant benefits in terms of both thermal comfort and
thermal energy efficiency in buildings. Limitations occur from limited window openings, shading,
reflection, and insufficient thermal storage. Typically, the thermal mass needed to store energy for a
domestic dwelling for longer than a day exceeds the solar capture capability of the glazed area of the
building. There is a balance between optimising solar access, excessive heat generation, and
excessive heat loss through windows.
This research investigates an alternative, active solar heating method, by the use of solar heated air
captured from the roof surface and blown through a sub-floor gravel thermal storage bed. The aim
of the research is to investigate a method of increasing the renewable fraction of the building
thermal energy supply by increasing the solar collector area and the thermal mass, and hence
thermal energy storage capacity. The hypothesis is that it should be possible to optimise thermal
energy storage to site specific conditions, balancing the thermal storage to the thermal load and
solar energy resource.
A prototype building has been constructed to research solar air collection and thermal storage
possibilities.
Keywords
Solar design; active; air; heating; collector
Introduction
Space heating forms a significant part of residential energy use in New Zealand (EECA/CAE, 1996).
Although New Zealand has a good solar resource (MED, 2010), it is a not a dominant method of
space heating (BRANZ, 2006).
Passive solar design has significant benefits in temperate climates (Boyle, 1996), however common
disadvantages occur from having to balance summer overheating with collector (glazing) area: as
glazing area is increased, so to does the risk of overheating. There are also economic considerations
balancing the cost of window area (normally double glazed) against the energy saving benefits.
A possible solution to passive solar disadvantages is the adoption of a hybrid active-passive design in
which an active solar collector supplements the passive design. Active solar air collectors have
received some research to date in New Zealand although it appears to be mainly related to
proprietary and wall systems (BRANZ, 2006). As with domestic hot water systems, there is a need to
produce systems with short payback periods if large-scale uptake is to be achieved. It therefore
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seems logical that integrated building solutions (with inherent reductions in relative installation cost)
provide a good starting point to achieve this goal.
This paper describes the initial steps taken in a project researching a novel solar air collector with
forced heated air movement through a pebble bed thermal storage unit.
Background
The author has designed and built a small family dwelling generally along low energy principles, in
the Totarabank development just outside Masterton in the Wairarapa (www.totarabank.com ). The
design was modified to include an underfloor pebble bed as a thermal store. The pebble bed is
heated by solar heated air, collected from a purpose built roof collector. The building is oriented
some ten degrees off north, and has unequal roof slopes to provide increased roof angle on the
north face for solar collection, yet also minimising the finished height by off-centring the apex.
The construction is conventional timber-framed with 150mm studs for extra wall insulation, plywood
cladding, and a coloursteel skillion roof. The roof construction uses two insulation layers: firstly
running vertically in between dummy rafters, and secondly running transversely over the dummy
rafters between oversized purlins. The floor is 125mm thick concrete slab with a blockwork
foundation wall. The blockwork wall has 100mm thick polystyrene perimeter insulation
approximately 300mm in depth. The polystyrene insulation (from Polypalace, Porirua) was the first
batch of 100% recycled polystyrene insulation in New Zealand.
The roof solar collector design was constructed with the goals of:
•
•
•
Simplicity
Low cost per unit area
Utilising readily available materials
The pebble bed, distribution piping, collector, and slab temperature sensors have been installed.
Ducting has been included in the under-slab to allow temperature sensors to be fitted at the
soil/pebble bed interface.
Only cursory monitoring has been carried out to date, as the project requires further funding for
temperature sensors and data logging equipment.
Methods
The key methodology steps proposed were:
•
•
•
•
•
•
Install air diffuser, pebble bed, solar collector, and temperature sensors.
Measure flow and temperature of air delivered to the pebble bed.
Measure the temperature of the exhaust air.
Calculate thermal energy lost by air (and gained by the thermal store).
Calculate the coefficient of performance – the energy use of the fan compared to the heat
energy gained by the slab.
Monitor the slab temperature at different locations to investigate heat movement and slab
temperatures.
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1.
3
2.
3.
1
10
4.
5.
2
6.
4
7.
8.
9.
10.
11.
8
9
11
7
5
6
13
12.
12
Figure 1 System schematic
13.
Clearlite transparent
roofing material.
Karaka colousteel roof.
Ridge cap and collector
pipe.
Fan.
Perimeter insulation
300mmx100mm.
Diffuser pipes through
gravel bed.
Blockwork foundation
wall.
Exhaust pipe.
Concrete slab.
Solar radiation.
Slab temperature
sensor.
Ground/gravel
interface temperature
sensor.
Underside of gravel
bed
1. Underfloor construction
A flexible, slotted drainage pipe was used as a distribution method for the heated air (fig. 2),
providing a cheap solution, possibly at the
expense of a more efficient, calculated ducting
system. The success of this aspect will become
clear once monitoring of the slab temperature
distribution has been carried out.
Figure 2 perforated air ducting and temperature sensor
ducting
Temperature sensor locations
(shows 7 of 9)
Given the depth to groundwater (>1.5m) and soil
type, and the desire for a significant pebble bed
thermal store, a perimeter insulation method was
selected (fig. 3).
This 100mm thick expanded polystyrene
insulation was installed over a 300mm depth
around the entire perimeter. The thickness was
selected by default, as this was the
manufacturer’s first attempt at 100% recycled
polystyrene (the manufacturer was PolyPalace, Porirua).
A thickness of 25 to 40mm is more usual.
Air diffuser pipes
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Nine ducting pipes were inserted at this point to allow later
installation of temperature sensors at the subgrade/pebble
bed interface. It is intended that this will facilitate three
dimensional temperature modelling throughout the thermal
store area.
Polystyrene perimeter insulation
Pebble thermal store (partially
installed)
A 6mm pebble was used as the primary thermal store media
(fig. 3): the density is not dissimilar to other particle sizes,
and any subsequent deviation from optimum thermal
properties has been relinquished in favour of economics.
6mm pebble is effectively a waste product in the region and
costs a quarter of 12mm or 20mm pebbles.
Figure 3 partially installed perimeter insulation and pebble infill
Temperature sensor probe in
ducting
Temperature sensors were included within the
concrete floor slab in locations above the nine
subgrade sensor locations (fig. 4). Sensors were
tied to the slab reinforcement, hence located at
approximately mid-slab.
Figure 4 temperature sensor within the concrete floor slab
2. Roof collector
A roof collector was considered preferable to wall collector as winter the winter sun angle at this
latitude (approx. 41 degrees south) favours roof mounted collectors where the roof angle is over
twenty degrees. Whilst the winter solstice midday sun altitude angle is 26 degrees, this rises to 29
degrees in the peak heating month of July, and averages 35 degrees over the heating season
(Duncan, 2005). The 35 degrees altitude angle is also close to the average solar day altitude angle for
August (Duffie&Beckmann, 1991).
Fifty percent of the heating energy occurs within two hours either side of mid day, beyond which
decreasing returns are achieved (Duncan, 2005), not including losses for increased incidence angle
in the horizontal plane. This corresponds to the higher altitude angles of the day.
A simple table showing incident angles on a pitched roof relative to solar altitude angle indicates
that for an altitude angle of 35 degrees the incident angle for a 35 degree roof pitch (as adopted on
the collector face of the building) is 20 degrees, some 15 degrees closer to the normal than a vertical
face (table 1).
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Table 1 Incident sun angle on various roof pitches relative to a vertical wall
Table of incident angles
roof pitch
sun altitude angle
5
10
15
20
25
30
35
40
45
50
55
60
vertical wall
25
60
55
50
45
40
35
30
25
20
15
10
5
30
55
50
45
40
35
30
25
20
15
10
5
0
35
50
45
40
35
30
25
20
15
10
5
0
-5
40
45
40
35
30
25
20
15
10
5
0
-5
-10
45
40
35
30
25
20
15
10
5
0
-5
-10
-15
50
35
30
25
20
15
10
5
0
-5
-10
-15
-20
55
30
25
20
15
10
5
0
-5
-10
-15
-20
-25
60
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
65
20
15
10
5
0
-5
-10
-15
-20
-25
-30
-35
70
15
10
5
0
-5
-10
-15
-20
-25
-30
-35
-40
75
10
5
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
25
30
35
40
45
50
55
60
65
70
75
Figures in red indicate combinations of roof pitch and altitude angle that favour pitched roofs over
wall mounted collectors (i.e. a lesser angle of incidence).
In addition wall collectors, by their nature, are generally subject to more restrictive shading than roof
collectors, particularly if summer overheating is an aspect considered during the building design
(giving rise to greater eave projections). The key driver of this research is the need to increase
renewable energy use by adoption of solar systems with a high coefficient of performance: Where the
efficiency is reduced by aspects such as shading the subsequent increase in payback time reduces the
likelihood of large scale uptake, and hence the proportion of renewable energy.
The roof collector is formed by providing a double-skin roof (fig. 5), with a clear polycarbonate outer
skin over a Karaka coloursteel corrugated roof.
Clear Polycarbonate
corrugate
Figure 5 Collector cross-section
12mm foam spacer
Colousteel corrugated
roof
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Separate solar water
collectors
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Red dashed line indicates doubleskinned roof collector area
Figure 6 Double skin roof solar collector
The collector glazing used was standard clear polycarbonate corrugated roofing material. A 12mm
gap was introduced by way of a spacer. The bottom end of the glazing is unsealed; the top end is
closed in by the roof ridge cap. Within the ridge cap, a polythene membrane was used to trap heated
air, which is ducted along the ridge in 100mm pipe. The pipe is insulated along its length down to the
building space. A temperature sensor was installed inside the air duct at the ridge location, and the
sensor wire threaded within the pipe down to the control unit.
The building is not only a prototype, it is a private dwelling, and it is important to incorporate the
flexibility to remove the collector if unsuccessful. The efficiency of a collector depends on a number
of climatic and constructional factors (Duffie & Beckmann, 1991). Efficiency is improved by glazing,
which acts to reduce convective heat losses, but also to provide a ‘greenhouse’ effect by allowing
solar wavelength radiation in whilst reducing the re-emitted radiation from the collector plate as it is
primarily of different wavelength. The solar transmittance of this material has not been analysed at
this point and was not a key factor in its selection. Once again ease of installation and cost formed the
primary criteria.
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The roof colour is ‘Karaka’, a deep matt green, and forms
the collector surface. No attempt was made to increase
solar absorption by installing a black, or specifically
constructed absorber surface, as this could detract from
aesthetics and might only provide limited returns as the
existing roof colour is at the higher absorption end of the
spectrum.
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Air ducting pipe
controller
The pipe passes through the ceiling cavity and into the hot
water cupboard on the first floor. Here a solid pipe joins to
and replaces the flexible pipe used in the ceiling cavity.
The pipe passes through the floor into the workshop space
below (fig. 7). This section of pipe has not yet been
insulated, but there is intention to do so.
Figure 7 Duct and controller components
In-line fan
Temperature sensors
At ground level within the workshop, an in-line fan was installed to draw heated air down and to blow
it through the pebble bed. The fan is operated by a controller (fig. 8) that senses the temperature at the
ridge and at a sensor temporarily inserted into one of the subfloor ducts. The controller is a standard,
basic, solar water controller and has editable settings governing the pumps/on/off criteria. It was used
in this case as if the fan were a solar water circulation pump: when the temperature in the ridge (T1) is
greater than the selected fan-on temperature (T2) the fan starts. When T1 drops below the fan-off
temperature (T3) the fan is shut off. The controller incorporates a ‘setting’ temperature (T4) – in this
case the slab temperature (T5). When the slab temperature reaches T5, a fan-off override applies
(table 2).
Table 2 Fan control parameters
Condition
T1>T2
T3>T1
T4=T5
Control command
Fan on
Fan off
Fan off
These parameters are selectable, and any combination can be applied.
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Figure 8 Controller showing ridge temperature at time of photo as 50.2 degrees C.
Results
Funding has not yet been sourced to complete the monitoring for this project. The infrastructure
components are in place and were tested to a basic degree by using a common low-wattage ventilation
fan.
Since installation of the ridge temperature sensor at the beginning of March 2010, the peak air
temperature from the collector was a little over 70 degrees C. General observation has shown
(between the beginning of March and 19th April) that on clear days ridge temperatures average
between 60 and 70 degrees C. There has been no structured data collection as yet, and no observations
during winter conditions.
Conclusion/Discussion
The project attempts to balance increased solar energy capture with a straightforward building
approach, thus whilst there may be more efficient methods in some instances, these may have been
subject of a compromise so as to ease the building consent process and reduce installation costs. Key
design parameters were to establish an affordable solar heating alternative to existing systems.
The ridge temperature achieved indicates that it is likely to be possible to providing heating on sunny
days, although the extent and efficiency of the system cannot be estimated without further monitoring.
Since installation (March 2010), there have been several frosts, followed by clear sunny days. The
temperature at the ridge on these days was in the 60-70 degree range, indicating that for at least some
of the year there is a useable resource coincident with heating load. It is interesting that research in
New Zealand (Heinrich, 2007) for existing solar wall collectors note peak differential temperatures of
around 40 degrees. This current research when completed will draw comparisons against such
existing systems.
The combination of convective opposition (trying to draw hot air downwards) and under slab pressure
losses (trying to blow air through the gravel bed) appear to have restricted the flow rate through the
underfloor distribution system, however the fan (a typical bathroom extractor fan at 20 Watts) is not at
all designed for this purpose. It is envisaged that a more substantial fan, preferably with variable flow,
would yield better results.
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The solar collector was constructed of standard, off-the-shelf domestic construction materials. The
overall cost of components was around $1500 (excluding monitoring equipment), yet resulted in a
collector area of some 24m².
Effective discussion of performance cannot be attempted until extensive monitoring is carried out.
This in turn is reliant on further funding.
Should monitoring results prove successful in terms of heat capture and transfer, it is envisaged that a
viable economic system would result.
References
Boyle,G 1996. Renewable Energy. Power for a Sustainable Future
BRANZ, 2006. Study Report SR155 (2006) [HEEP executive summary]
Duffie & Beckmann, 1991. Solar Engineering of Thermal Processes.
Duncan, A.G. 2005 Selected Solar Design Tools for Residential Development, Master of Engineering
Thesis, Massey University
EECA/CAE, 1996 New and Emerging Renewable Energy Technologies
Heinrich, Matthias. 2007. ‘Transpired Solar Collectors – Results of a Field Trial’. BRANZ Study Report
167. BRANZ Ltd, Judgeford, New Zealand.
MED, 2010 Minstry of Economic Development,
http://www.med.govt.nz/templates/MultipageDocumentPage____40710.aspx