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Innovative alternative solar thermal solutions for housing in conservation-area sites listed as national heritage assets
Energy and Buildings 89 (2015) 123–131
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Innovative alternative solar thermal solutions for housing in conservation-area sites listed as national heritage assets C. Cristofari a,∗ , R. Norvaiˇsiene˙ b , J.L. Canaletti a , G. Notton a a b
UMR CNRS 6134 Scientific Centre Georges Peri, University of Corsica – University Institute of Technology, Route des Sanguinaires, F20000 Ajaccio, France Institute of Architecture and Construction of Kaunas University of Technology, K. Donelaiˇcio g. 73, 44249, Kaunas, Lithuania
a r t i c l e
i n f o
Article history: Received 1 November 2014 Received in revised form 5 December 2014 Accepted 21 December 2014 Available online 29 December 2014 Keywords: Solar energy Thermal collectors Building integration
a b s t r a c t In a privatized global marketplace, innovation is a key driver of sustainable development and national competitiveness. Here we report on an innovative new fully building-integrated thermal solar panel concept that is patented and currently being readied for commercialization. The paper outlines the French regulatory landscape governing the deployment of thermal solar panels in France, and thus the need for countries like France to develop new building-integrated solar power meeting “building-envelope” integration requirements for protected areas. We go on to introduce the new as-developed system, its physical modelling via a finite element analysis model constructed using an electric circuit analogy, and the results achieved on a detached home retrofitted with this system and trialled for a 12-month period. This paper leads out of COST – European Cooperation Science and Technology – framework action TU 1205 “Building Integration of Solar Thermal Systems”. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Building-sector energy demand is rising fast in many countries. In France, 30 million homes use around 50% of the energy budget and produce 25% of national greenhouse gas emissions. In Europe, the building sector accounts for around 40% of total energy demand, which makes improving building energy performances crucial in order to meet EU energy efficiency targets and combat global climate change – and create a platform towards domestic energy security. In 2002, the EU introduced the Energy Performance of Buildings Directive (EPBD) giving EU States an integrative methodology-based approach designed to foster efficient energy use across the building sector. The EC then recognized that there was non-negligible potential for profitable energy savings still going unexploited, which is why in November 2008 it proposed a recast of the EPBD designed to enable energy savings amounting to a further 60–80 million tonnes of oil equivalent per year to 2020, i.e. a further 5–6% reduction in total EU energy consumption, over and above the previous maximum achievable if the recast directive was integrally implemented across the board. Based on these proposals, in April 2009 the European Parliament passed a
∗ Corresponding author. Tel.: +33 495 56 8375; fax: +33 495 524 142. E-mail addresses: [email protected] (C. Cristofari), ˙ [email protected] (J.L. Canaletti), [email protected] (R. Norvaiˇsiene), [email protected] (G. Notton). http://dx.doi.org/10.1016/j.enbuild.2014.12.038 0378-7788/© 2014 Elsevier B.V. All rights reserved.
legislative motion calling for even tougher more ambitious legislation. The EU Council now needs to reach a position on this strand of proposals. In the meantime, the tertiary and housing sector remains the leading energy consumer in France, at 69.04 Mtoe. Percent of market demand is stable at 43%, but the absolute figures jumped +25% between 1973 and 2008 [1]. Every day, an average EU citizen uses around 35 L of domestic hot water heated to 60 ◦ C, and the trend is an upward curve, as the energy needed to produce domestic hot water has crept up over the years as people have progressively got used to greater indoor comfort. Heating (at 50%) and water heating (25%) are clearly the biggest subsectors in terms of energy consumption (Fig. 1). For lowenergy buildings, water heating and space heating account for 50% and 30% of energy consumption, respectively, with the remaining 20% down to electrical home appliances [2]. Using renewable energies can both improve energy efficiency and reduce fuel or electricity consumption. The chosen solution must be able to fit new or recent housing and, more importantly still, to retrofit old housing stock, which the biggest energy consumer. Solar collectors are a particularly well adapted option for use in this sector. A solar power system can efficiently cover up to 80% of household domestic hot water needs – and all with the added advantages of less fuel used, minimal pollution and little maintenance effort. The thermal solar market is booming, with 2008 figures pointing to a 51.5% jump on 2007 and over 4.5 Mm2 installed
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Electrical auxiliaries, 25% Heang Heang, 50%
Water Heang Electrical auxiliaries
Water Heang, 25%
Fig. 1. Breakdown of housing-sector energy consumption in France.
(3172 MWth) [3]. There has been a fresh surge in research into thermal solar, which is explained by a combination of factors: • Increased energy use tied to producing domestic hot water. • Development of sustainable new solutions for cutting building energy use. • A booming thermal solar market. Introducing innovative and environmentally friendly solutions is a vital task made complicated by the number of bottlenecks involved – some financial, others technical, psychological, and even legislative (building code). The solutions proposed need to be seamlessly building-integratable and minimize eyesore (psychological bottleneck). They also need to be easily installed onto new building and old housing stock (technical bottleneck), not too expensive (financial bottleneck), and environmentally friendly. Our basic idea, in a nutshell, is to ‘activize’ passive components of a build. 2. Thermal solar collectors and the governing French legal landscape From the outset, installations harnessing solar energy have been tarnished by a reputation for creating an eyesore, which has turned many potential users away. On the aesthetics front, photovoltaic systems already offer households and designer large room for manoeuvre, as the cells are small and modular and the electrical cabling is far more flexible to work with than regular pipework, which substantially increases their potential integratability [4,5]. Outside of the sociological dimension – which remains a complex focus of study – there are legislative roadblocks based on heritage asset conservation policy, typically: • Heritage protection perimeters surrounding a historic-interest monument; • Conservation-area districts; • ZPPAUP – conservation areas for ‘Protected Urban, Architectural and Landscape Heritage’. Article 28 of French act no. 2010-788 dated 12 July 2010 was reformed to replace ZPPAUP with AVAP – areas of heritage value; • Listed or scheduled heritage sites. An AVAP project is purposed with the development and enhancement of built-environment heritage and historic spaces, as an extension of committed sustainable development policy. Any construction or development project inside these areas must first apply for planning permission from the STAP – the local-region buildings, monuments and heritage assets authority. An ABF – Architecte des Bâtiments de France, the appointed advisor charged with supervising that any new build or development work bordering onto protected monuments will not create an eyesore – then
issues their local authority with a straight consent, development consent needed or full consent procedure [6]. Unfortunately, solar power facilities do not qualify as “showcasing heritage value”. Furthermore, the ABF also have an influence that stretches beyond AVAPs, as their scope of authority extends to: • The surroundings in the vicinity of historic monuments and listed heritage sites: a protected-area perimeter spanning the 500 m radius around any historic monument [7,8]. This authority was enacted by the French Planning Act of 1943, which stipulates that “any home located in line of sight of a historic monument may not be extended, demolished, cleared of trees or altered in any way that might affect its character” [7]; • Scheduled sites: sites governed by the French Ministry for Sustainable Development, which missions the ABF out to the ground [8]; • Conservation-area districts: the ABF supervises that any first-fix (building envelope) or second-fix (structural interior) remodelling is compliant with the preservation and rehabilitation area map (Malraux Act of 1962 safeguarding historic-interest town centres); • AVAPs: the ABF coordinates the study then checks the projects are code-and-regulations-compliant. Taken together, these areas add up to represent huge potential for working thermal solar energy. As of 31 December 2012, France counted 43,180 monuments, i.e. 14,367 ‘listed’ and 28,813 ‘scheduled’, registered as holding historic interest [9], and the list is getting longer every year, plus the hundred-odd conservation-area districts that cover an average surface area of 6600 ha each [9]. That said, there are signs the legislature is loosening up: • Under round 1 of the multi-party ‘Grenelle Environment’ reform, the French Senate and National Assembly, initially divided on the bill, finally agreed to a measure voted in Thursday 23 July 2009, passed by a special joint government – senate committee. Under this measure, ABFs saw their powers clipped back, as the duty to secure ‘permitted development’ consent was replaced by a straightforward ‘consultative’ ruling in the vast majority of cases. Today, the decision taken essentially follows local zoning code, called PLU – Plan Local d’Urbanisme. In practice, to determine the impact of this kind of measure, it would be necessary to pore through every single ‘PLU’ zoning map of France, as each one is defined at local council level (or, in rare cases, at cross-localcouncil level); • The French Act no. 2010-788 dated 12 July 2010, dubbed ‘Grenelle Environment Round 2’, amended the French Urban Planning Code by ushering in article L.111-6-2, under which paragraph 1 reads: “despite any town planning provisions to the contrary, planning permission may no longer oppose the installation of equipment that produces renewable energy to cover the household energy needs of the occupants of all or part of the building”. Table 1 pools and recaps the various administrative procedures governing project sites according to area location. This short analysis highlights how the stiff French homes and housing legislation privileges aesthetics over energy performance. Although there are signs the legislation is softening up, the conditions imposed are still strict and, in tandem with the financial grants and incentive schemes available, tend to support a policy of maximal integration of energy supply components. Indeed, government directives further anchor this policy priority given to building-integrated solar technologies to foster more landscape/architecture-conscious aesthetics and to position home solar trades and industries on a higher-value-added subsector [11]. Solar energy is coming to town and shaking up the old building
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Table 1 Recap of the procedures applicable to residential build projects according to area location [10]. In heritage protection perimeter surrounding a historic-interest monument
In a conservation-area district
ZPPAUPa /AVAPb
In a scheduled heritage site
In a listed heritage site
- Straight consent when the building is in line of sight of a historic monument - Development consent needed in line of sight of a historic monument
- Development consent needed
- Full consent procedure
- Straight consent - Development consent needed for permission to demolish
- Straight consent with consultative input from the local-region nature, sites and landscapes commission
a b
ZPPAUP – conservation areas for protected urban, architectural and landscape heritage. AVAP – areas of heritage value.
Fig. 2. Illustration of the H2OSS concept installed in situ.
function and design codes. Under this new paradigm, solar power facilities are now energy-producing features of the building envelope. French Thermal regulation 2012 accelerates the process of solar installations acquiring as can do for French tropical island (Reunion) the approach of global quality standards for natural and low energy cooling in buildings [12–14]. 3. Outline of the new solar collector concept The H2OSS flat-plate thermal solar collector concept, as developed and patented [15], was devised as a solution to tackle the above-cited bottlenecks via a strategy based on activizing housing components that have traditionally remained passive. The solar
roof ledge was designed concomitantly with our lab-developed airbased solar shutter system which is equipped with a hybrid PV – thermal solar collector [16] for home space heating. Developed to the fully building-integrated design brief, the H2OSS thermal solar collector is a flat-plate collector designed as a forced-circulation home solar water-heating system. The innovation lies in its long slim geometry enabling it to slot into a pre-profiled aluminium gutter channel used for rainfall evacuation. This aluminium gutter channel profile can be built in place, in one piece (running up to 20-odd metres long), using the industrial extrusion process. The gutter channel obviously retains its rainfall evacuation role. The gutter channelling comes in an array of styles and colourways
Fig. 3. The system installed at the Vignola Lab (Corsica) – overhead view and cross-section view of the H2OSS solar collector.
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Fig. 4. System of fixtures fastening the modules inside the gutter channel.
for enhanced building integration. These modules are quick and easy to install, and the installed system is invisible from ground level, as the main circuit inflow and outflow tubing is hidden away inside the rainfall evacuation tubes (Fig. 2). The H2OSS solar module draws on the most tried-and-tested technology for this type of collector, i.e. a semi-transparent glazed cover topsheet, a highly selective absorber plate, a heat-insulating backsheet, and two copper tubes that recover the heat energy converted by the absorber (Fig. 3). An installation is comprised of a set of modules interconnected in series and/or parallel circuits. Each module measures 1 m long by 0.14 m wide and is connected to the next by a split-point junction making it possible to swap a given module in or out, as necessary. The tube located just under the fin is the outflow tube, and the other is the inflow tube containing the coolant that recovers heat from the absorber. Fig. 4 shows the space that needs to be left between module and gutter channel in order to evacuate rainwater. Note also that the absorber is edged with a pair of folds called ‘fins’. The job of these fins is to increase the captation surface of the otherwise relatively small modules, and trials with prototypes have validated the role of these ‘fins’. The collectors are very easy to fit: • There are no leaktightness issues to deal with, as the collector is no longer in-roof; • The solar system installer no longer needs roofer training – plumbing skills are all that is needed; • The installation is a modular design making for convenient transport and handling; • The modules are fastened onto the gutter channel via fixtures that are invisible from ground-level (Fig. 4). Module tilt can be adjusted to fit each installation configuration; • The inside workings of the installation complete with tubing and storage tank is identical to the current generation of home solar water-heating systems.
This collector is designed preferably to be mounted south-facing but can, depending on site configuration and roof pitch, be installed on north-facing buildings. There is no particular servicing or maintenance required, other than greater attention if the equipment installed is surrounded by deciduous trees. The gutter channels may need clearing or cleaning once or twice a year. The collector itself will suffer relatively little from dust or dirt build-up, and the occasional rain shower is generally enough to keep the system performing to within a few percentage points of peak transmittance [17].
4. Concept characterization, model and validation 4.1. Defining parameters of the solar collector We previously tested the solar collector’s thermal behaviour to validate the thermal model [18] and attempt to improve performances [19] by trialling certain parameter adjustments (Fig. 5). Briefly, the trials were led on a site at our laboratory in the Gulf of Ajaccio (latitude: 41◦ 55 N; longitude: 8◦ 55 E) at an altitude of 70 m asl and about 200 m from the seafront. This trial setup enabled us to operate closer to European Standard EN 12975-1 [20], with four rows of four modules (2.24 m2 ) connected in series and fixed on a solar tracker for better control of solar intensity and direction. The solar modules are connected to a thermal loop that regulates the input fluid temperature by heating the fluid if it is too cold or cooling it via an air cooler if the fluid is too hot. Measurements were taken on each module at 60 s intervals to record: solar irradiance on the collector plane (using a Kipp & Zonen CM11 pyranometer), ambient temperature, humidity, wind speed and direction, fluid flow rate and input and output fluid temperatures. This experimental trial enabled us to plot the performances of the new solar collector. Performance efficiencies were calculated experimentally for various measured reduced temperatures, and linear regression was applied to determine optical efficiency and thermal losses according to Eq. (1): = −KTr + B
with Tr =
(Tm − Tamb ) I0
(1)
where I0 is solar irradiance, Tm is mean temperature, Tamb is ambient temperature, B is optical efficiency (dimensionless) and K represents the thermal losses (W m−2 K−1 ) [21].
Fig. 5. Experimental setup: solar tracker fitted with H2OSS modules.
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Fig. 6. H2OSS® solar collector design as electric circuit analogy.
Table 2 Optical efficiency and thermal losses. H2OSS Optical efficiency, B Thermal losses, K (W m−2 K−1 )
0.903 13.80
Table 2 reports the optical efficiency and thermal losses. While our optical efficiency was high, the coefficient relative to thermal losses was bad. This difference is due to the geometry of the H2OSS® modules, as thermal losses are heavier on the sides of the modules and so performances decrease rapidly when reduced temperature increases. The H2OSS collector performed better with a low reduced temperature and worked better when input water temperature was low, which prompted us to use a water storage tank with thermal stratification making the system work at low flow rate so that the water coming from the tank to the solar collector will be colder.
1 2
3 Fig. 7. Close-up of the first node of the glazed flat plate.
+
4.2. Thermal model 4.2.1. H2OSS numerical model We developed a two-dimensional thermal network model [18] based on nodal discretization method. The solar collector is broken down into 97 finite-element volumes that are assumed to be isothermal, and for each node, we describe the governing thermal equilibrium equation based on an electric circuit analogy (Fig. 6) in which temperatures, sources of flow and boundary temperatures are assimilated to electric potentials, currents, current generators and voltage generators. The thermophysical properties of the component collector materials are assumed to be constant within the effective range of working temperatures. The input parameters used in the model are solar irradiance I0 , ambient temperature Tamb , airspeed over the front face of collector V, temperature at ground level Tground , temperature overhead Tsky , and temperature of the coolant fluid at the collector in-feed. To illustrate, the equation corresponding to the first node of the glazed flat plate (Fig. 7) is detailed in Eq. (2). glass · Cpglass · V4 ·
dT4 4 = S4 · ˛glass I0 −εglass · S4 · f4−sky · (T44 −Tsky ) dt
2
+
Tamb − T4 T3 − T4 T5 − T4 + + Rcond + Rconv Rcond Rcond
n
εglass · Strans2 · f2−n · (T14 − Tn4 )
0
+
T20 − T4 Rcond + Rconv
(2)
where S is surface, ˛ is attenuation coefficient, I0 is solar irradiance, ε is coefficient of emissivity, f is form factor, T is temperature, and R-value is thermal resistance and n multiple reflections in the air layer. All three types of heat transfer are mapped as thermal resistances. Higher R-values mean lower heat flux values. Rcond corresponds to a conductive resistance that is dependent on thermal conductivity of the material, thickness e to be crossed, and heat transfer area S. Rconv corresponds to a convective resistance that is dependent on heat transfer coefficient h. Radiative transfer was expressed using the linearizing Stefan–Boltzmann equation that can translate radiative transfers into thermal resistances. However, for sharper accuracy, we used the non-linear Stefan–Boltzmann equation, which means thermal equilibrium is expressed as stated in Eq. (2).
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For the equation governing energy balance of the fluid circulating inside the copper tubes, the temperature at the coolant tube wall was estimated using the NTU method (Eq. (3)) that produces an estimate of the temperature profile of a fluid circulating inside a homogeneous tube (Ta ) for an internal surface area Sfc under steady-state flow [22]. Tfc/s = Ta + (Tfc/e − Ta ) e−NTU
(3)
where NTU is number of transfer units (Eq. (4)). NTU =
hfc/a Sfc ˙ fc Cpfc m
(4)
Using Eqs. (3) and (4), and knowing the inflow temperature of the fluid and the mean tube temperature, it becomes possible to calculate fluid temperature at the collector outflow. The 97 equations – one for each of the 97 finite-element volumes – were solved numerically. The model thus obtained is resolved using an implicit direct integration method. 4.2.2. The hydraulic loop During the experimental phase, we noted that one of the key problems with our solar collector is hydraulic resistance due to the collector’s linear structure. Consequently, it would be wise to keep this solar system working in low flow-rate conditions. By reducing hydraulic losses, the low rate regime also brings other advantages, as reported in [23]. Thermal stratification: using low-flow operation results in an increased solar collector outlet temperature and consequently induces a higher degree of thermal stratification inside the tank Moreover, water temperature at the top of the tank will be closer to setpoint load temperature, which means auxiliary energy consumption will be decreased, thus increasing solar fraction. Furthermore, with highly stratified heat storage, the return temperature to the solar collector will be lower and the working periods for the solar collector will be longer, which should lead to increased net energy output from the solar collector [24,25]. - Piping in the solar collector loop: low-flow systems can make use of smaller pipes, which means less material needed for pipes and insulation thus less heat losses. - Pump: the energy demand of the circulation pump is decreased, which is a critical factor in our case. The thermal loop behaviour is simulated using a numerical code based on a nodal approach [23–27]. It is divided in 19 nodes: 7 for fluid circulation, 10 for the storage tank (optimal number of nodes to optimally take into account the thermal stratification [23]), and 2 for the water storage inlet and outlet (see Fig. 8). The temperature of the water at the outlet of the solar collector and the average temperature of the solar absorber are obtained by modelling the solar collector, keeping in mind that all 97 temperatures in the solar collector need to be computed to determine these two temperatures. The energy balance, in 1D, is applied and an iterative method is used to solve the first-order differential equations. A reversionelimination mixing algorithm based on a thermal mix of certain storage tank nodes to obtain a correction factor giving a positive temperature gradient from bottom to top of the tank [28,29] was used to simulate in-tank thermal stratification. The tubes between the tank and the solar collector are 9 m long, with 3 m inside the building (ambient temperature is thus the temperature inside the building – thermal losses are thus taken into account). The coil heat exchanger is modelled by five nodes, and thermal power between the heating fluid and the water into the tank is calculated using the NTU method [30].
Fig. 8. Model node definitions [6].
Table 3 Accuracy of the thermal model for the nine temperature points inside the solar module.
RMSE (◦ C) RRMSE (%)
T1
T2
T3
T4
T5
T6
T7
T8
T9
1.65 9.4
1.73 6.4
1.69 4.6
2.36 8.3
2.05 6.1
1.98 5.8
1.39 9.4
1.21 7.9
1.09 8.2
4.3. Experimental validation With implementation of the thermal model, we moved on to validate the model based on experimental data collected with the second experiment (including a solar tracker) [18]. A thermal solar module was specially instrumented with nine thermal sensors (type PT100 class B) measuring surface temperature at nine specific points (Fig. 9): (1) On the glass; (2) on the right fin; (3) on the left fin; (4) on the right absorber; (5) on the left absorber; (6) in the right insulation; (7) in the left insulation; (8) on the right side and (9) on the rear face. The solar thermal modules are connected in parallel delivering a water flow rate of 450 L h−1 ; in these conditions, the increase in water temperature is small but the temperature gradient inside the module is more clearly visible. An experimental verification was carried out on the all-year data. The setup demonstrated good accuracy between modelled and experimental temperatures. We calculated the absolute and relative root mean square errors (RMSE and RRMSE) over the total end-to-end experimental period and over four seasons (about 100 days). RMSE and RRMSE values for the nine temperatures calculated from Eqs. (5) and (6) are given in Table 3.
RMSE =
n (X i=1 sim,i
RRMSE =
− Xexp,i )2
(5)
n
n [(Xsim,i i=1
− Xexp,i )/Xsim,i ] n
2
(6)
where n is number of data and Xsim,i and Xexp,i are respectively simulated and experimental data. The simulated and experimental temperatures show fairly good fit. We also simulated output water temperature with the thermal module connected in series and delivering a water flow rate of 140 L h−1 . These conditions were chosen to approach real operating conditions. We computed RMSE and RRMSE for output water temperature over four seasons (using about 90 days) and obtained the following values: RMSE = 1.4 ◦ C and RRMSE = 5.2%. These errors are fully acceptable. These two validations certify that the model is
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Fig. 9. (a) Temperature sensors positions, (b) temperature sensor, (c) and (d) instrumented thermal modules.
Table 4 Optical efficiency and thermal losses.
solar system in order to benchmark the energy performances of the installation. The installation set up comprises a 200 L electric–solar hot water storage tank with a 2 kW resistance. It features 20 serially connected H2OSS solar collectors covering a total working surface of 2.8 m2 . The main solar circuit comprises 9 linear metres of pipework between first collector and storage tank, including 6 linear metres that are outside the collector system loop. The installation runs at 80 L h−1 m−2 . Table 5 lists the recorded mean monthly demand-water temperatures and mean monthly outdoor temperatures. Fig. 11 charts the year-averaged over-day demand profile recorded for this detached home housing four occupants (two parents and two children).
H2OSS Optical efficiency, B Thermal losses, K (W m−2 K−1 )
0.780 6.25
robust to be used in future work studying the influence of changes in materials, structure, or other possible modifications. 5. Process of evolving this solar collector into an optimized version The solar collector was numerically optimized for a conventional installation in single-family housing in the south of France. We previously showed [19] that repositioning the cold water tube into the absorber rather than the insulation layer yields much better performances than with the current prototype. Thermal insulation and air layer thicknesses were then optimized, and the influence of water flow rate proved very high by virtue of the design of this new solar collector. Three configurations were studied (Fig. 10). The configuration that best optimizes performances is configuration ‘b’ with the two parallel tubes on the same plane. Table 4 gives the new computed optical efficiency and thermal losses of configuration ‘b’. This new configuration of the H2OSS concept will be simulated for a single-family house, and performances will be compared against the installed prototype.
6.1. Results of the installed running prototype We measured energy performances over a 12-month year (2013) in operation. This solar power installation delivers a solar fraction of 27%. Over the year 2013, we get a net solar power output of 835 kWh for a domestic hot water demand of 3055 kWh and a solar radiation of 4901 kWh received. Fig. 12 charts solar radiation received, domestic hot water demands, effective solar power output and solar fraction monthby-month over 2013. Note that over the trial year 2013, the installation showed energy losses on the main solar circuit (tubing) amounting to 93 kWh and losses via the electric–solar hot water storage tank amounting to 371 kWh. Annual energy consumption of the solar circuit pump amounts to 63 kWh.
6. Real-world performances of a solar facility located in southeastern France (Bastia, Corsica; Fig. 2) and simulated performances computed for this new concept
6.2. Simulated performances computed for a new-version H2OSS We simulated energy performances over a 12-month year (2013) in operation for the optimized new-version H2OSS solar collector. This solar power installation now delivers a solar fraction of 48%. Over the year 2013, we get a net solar power output
This building-integrated solar thermal system is located in Corsica, at latitude 42◦ 42 10 N, longitude 9◦ 26 59 E, altitude 51 m asl, in a Mediterranean climate zone. We instrumented the thermal
Table 5 Recorded mean monthly demand-water temperatures and mean monthly outdoor temperatures.
Demand-water temperatures Outdoor temperatures
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
8.6 9.2
8.2 9.5
8.8 11.9
10.5 13.5
12.6 18.9
14.7 22.8
15.9 24.9
16.6 25.8
15.8 20.8
14.2 17.6
12.0 12.8
10.5 10.0
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Fig. 10. Different studied configurations.
600 500 Solar Radiaon (kWh) 400 Domesc hot water demand (kWh) Effecve solar producon (kWh) Solar Fracon %
300 200 100 0
Fig. 13. Energy performance (K = 6.25/B = 0.78).
Fig. 11. Average daily profile for domestic hot water supply.
the
simulated
solar
thermal
system
7. Conclusion
600 500 Solar Radiaon (kWh) 400 Domesc hot water demand (kWh) Effecve solar producon (kWh) Solar Fracon %
300 200 100 0
Fig. 12. Energy performance (K = 13.80/B = 0.903).
of
of
the
existing
solar
thermal
system
of 1408 kWh for a domestic hot water demand of 3055 kWh and a solar radiation of 4901 kWh received. Fig. 13 charts solar radiation received, domestic hot water demands, simulated solar power output and solar fraction monthby-month over the 2013. This new version, as simulated over the trial year 2013, showed energy losses on the main solar circuit (tubing) amounting to 121 kWh and losses via the electric–solar hot water storage tank amounting to 429 kWh. Annual energy consumption of the solar circuit pump amounts to 64 kWh.
Despite extensive deployment of solar power worldwide, the solar economy still faces a number of bottlenecks. One of the takehome messages from an R&D seminar hosted by the ADEME – French Environment and Energy Management Agency at Sophia Antipolis (France) on 27 and 28 April 2004 to address “innovation needs for thermal solar power” was that the thermal solar systems on the market are essentially unintegrated “add-ons” to the building. The problems that emerged – both on the technical front and the aesthetics front – are patent bottlenecks to development. Furthermore, as the solar market has now gained relative maturity, solar product costs have begun to stabilize and are unlikely to show any fresh drop in the near future. The upshot is that short of any economies of scale driven by a boom in the market, only a disruptive technology causing a shift in design-engineering could spark a significant new shift in solar economics. The bottlenecks, in order of importance, are first financial, then technical, legislative, and finally psychological. It is in this context that we propose a new home-integrated solar power concept. This paper outlines the French legislative constraints governing the deployment of thermal solar collectors in France, before going on to introduce the new integrated concept by outlining the modelling process and model validation ready for use in simulations. We report a running prototype installed in Corsica, France, and its energy efficiency performances achieved over the year 2013. This solar power installation delivers a solar fraction of 27% for a net solar power output of 835 kWh. Simulations of a new optimized version of the solar collector prototype run for this same installation yielded improved results, with a solar fraction of 48% for a net solar power output of 1408 kWh – and consequently a net gain approaching 78%. This research addresses EU energy policy challenges tied to integrating
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thermal solar systems into buildings, and specifically the COST – European Cooperation Science and Technology – framework action TU 1205 “Building Integration of Solar Thermal Systems” (BISTS). References [1] Service de l’Observation et des Statistiques (SOeS), Le Point sur Bilan énergétique de la France en 2010; 91, 2012, Available at http://www.insee.fr [2] CEREN (Centre d’Etudes et de Recherches Economiques sur l’Energie), Consommation totale d’énergie des secteurs résidentiel et tertiaire, par usage, de 1990 à 2009, 2011, June, www.statistiques.developpement-durable.gouv.fr [3] D. Besson, Consommation d’énergie: autant de dépenses en carburants qu’en énergie domestique. ISEE Première; 1176, 2008. [4] I.B. Hagemann, Gebäudeintegrierte Photovoltaik, 2002. [5] T. Reijenga, What do Architects Need?, Switzerland, IEA-PVPS 7-03:2000, 2000. [6] Ministère du Développement Durable, 2012, http://www.developpementdurable.gouv.fr/ [7] Loi du 31 Décembre 1913, loi sur les monuments historiques, 1913. [8] Loi du 2 mai 1930, Règlementation parcs et jardins, espaces naturels, terroirs marqués par l’homme, 1930. [9] Ministère Franc¸ais de la Culture, 04-patrimoine-architecture-2010.pdf (Objet application/pdf), 2010, http://www2.culture.gouv.fr/culture/deps/chiffrescles2010/04-patrimoine-architecture-2010.pdf [10] Préfecture du Haut-Rhin, Guide pour l’installation de panneaux solaires thermiques ou photovoltaïques dans le Haut-Rhin. Préfet du Haut-Rhin. [11] Ministère de l’écologie, de l’environnement, du transport et du logement, Politique énergétique – Ministère du Développement durable, 2012, http://www. developpement-durable.gouv.fr/Politique-energetique.html [12] F. Garde, H. Boyer, J.C. Gatina, Elaboration of global quality standards for natural and low energy cooling in French tropical island buildings, Build. Environ. 34 (1) (1999) 71–83. [13] A.J.P. Lauret, T.A. Mara, H. Boyer, L. Adelard, F. Garde, A validation methodology aid for improving a thermal building model: case of diffuse radiation accounting in a tropical climate, Energy Build. 33 (7) (2001) 711–718. [14] H. Boyer, F. Garde, J.C. Gatina, J. Brau, A multimodel approach to building thermal simulation for design and research purposes, Energy Build. 28 (1) (1998) 71–78.
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[15] C. Cristofari, Device for Collecting Rainwater and Solar Energy Originating from Light Radiation, U.S. Patent WO 2006/100395 A12006. [16] J. Canaletti, Shutter Comprising a Built-in Air Solar Converter, Euro Patent EP 1709279 B1, PCT/FR2004/003089, 2007. [17] A.K. Soteris, Solar thermal collectors and applications, Prog. Energy Combust. Sci. 30 (3) (2004) 231–295. [18] F. Motte, G. Notton, C. Cristofari, J.L. Canaletti, A building integrated solar collector: performances characterization and first stage of numerical calculation, Renew. Energy 49 (2011) 1–5. [19] G. Notton, F. Motte, C. Cristofari, J.L. Canaletti, Performances and numerical optimization of a novel thermal solar collector for residential building, Renew. Sustain. Energy Rev. 33 (2014) 60–73. [20] European Standard EN 12975-1, Thermal Solar Systems and Components – Solar collectors – Part 1: General Requirements, March, 2006. [21] F.P. Incropera, D.P. DeWitt, T.L. Bergman, A.S. Lavine, Fundamentals of Heat and Mass Transfer, 6th ed., John Wiley, 2007, ISBN 978-0-471-45728-2. [22] F.P. Incropera, D.P. Dewitt, Fundamentals of Heat and Mass Transfer, 4th ed., Wiley, 1996. [23] C. Cristofari, G. Notton, P. Poggi, A. Louche, Influence of the flow rate and the tank stratification degree on the performances of a solar flat-plate collector, Int. J. Therm. Sci. 42 (5) (2003) 455–469. [24] L.J. Shah, Investigation and Modeling of Thermal Conditions in Low Flow SDHW Systems, Report R-034, Department of Buildings and Energy Technical University of Denmark, 1999. [25] S. Furbo, Optimum design of small DHW low flow solar heating systems, in: ISES Solar World Congress Budapest Report 93-24, 1993. [26] D. Haillot, F. Nepveu, V. Goetz, X. Py, M. Benabdelkarim, High performance storage composite for the enhancement of solar domestic hot water systems. Part 2: numerical system analysis, Sol. Energy (2012) 64–77. [27] D. Haillot, PCM for Optimization of Solar Thermal Collector: Materials and Process, PhD dissertation, University of Perpignan, France, 2009 (in French). [28] D. Mather, K.G. Hollands, J. Wright, Single- and multi-tank energy storage for solar heating systems: fundamentals, Sol. Energy 73 (2002) 3–13. [29] F. Nepveu, Décentralized Electricity and Heat Production by using Parabole/Stirling: Application to the EURODISH System, PhD dissertation, University of Perpignan, 2008. [30] K. Shah, A.C. Mueller, in: W.M. Rohsenow, J.P. Hartnett, E.N. Ganic (Eds.), Heat Exchangers, Handbook of Heat Transfer Applications, Mc Graw Hill, 1985.
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Innovative alternative solar thermal solutions for housing in conservation-area sites listed as national heritage assets C. Cristofari a,∗ , R. Norvaiˇsiene˙ b , J.L. Canaletti a , G. Notton a a b
UMR CNRS 6134 Scientific Centre Georges Peri, University of Corsica – University Institute of Technology, Route des Sanguinaires, F20000 Ajaccio, France Institute of Architecture and Construction of Kaunas University of Technology, K. Donelaiˇcio g. 73, 44249, Kaunas, Lithuania
a r t i c l e
i n f o
Article history: Received 1 November 2014 Received in revised form 5 December 2014 Accepted 21 December 2014 Available online 29 December 2014 Keywords: Solar energy Thermal collectors Building integration
a b s t r a c t In a privatized global marketplace, innovation is a key driver of sustainable development and national competitiveness. Here we report on an innovative new fully building-integrated thermal solar panel concept that is patented and currently being readied for commercialization. The paper outlines the French regulatory landscape governing the deployment of thermal solar panels in France, and thus the need for countries like France to develop new building-integrated solar power meeting “building-envelope” integration requirements for protected areas. We go on to introduce the new as-developed system, its physical modelling via a finite element analysis model constructed using an electric circuit analogy, and the results achieved on a detached home retrofitted with this system and trialled for a 12-month period. This paper leads out of COST – European Cooperation Science and Technology – framework action TU 1205 “Building Integration of Solar Thermal Systems”. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Building-sector energy demand is rising fast in many countries. In France, 30 million homes use around 50% of the energy budget and produce 25% of national greenhouse gas emissions. In Europe, the building sector accounts for around 40% of total energy demand, which makes improving building energy performances crucial in order to meet EU energy efficiency targets and combat global climate change – and create a platform towards domestic energy security. In 2002, the EU introduced the Energy Performance of Buildings Directive (EPBD) giving EU States an integrative methodology-based approach designed to foster efficient energy use across the building sector. The EC then recognized that there was non-negligible potential for profitable energy savings still going unexploited, which is why in November 2008 it proposed a recast of the EPBD designed to enable energy savings amounting to a further 60–80 million tonnes of oil equivalent per year to 2020, i.e. a further 5–6% reduction in total EU energy consumption, over and above the previous maximum achievable if the recast directive was integrally implemented across the board. Based on these proposals, in April 2009 the European Parliament passed a
∗ Corresponding author. Tel.: +33 495 56 8375; fax: +33 495 524 142. E-mail addresses: [email protected] (C. Cristofari), ˙ [email protected] (J.L. Canaletti), [email protected] (R. Norvaiˇsiene), [email protected] (G. Notton). http://dx.doi.org/10.1016/j.enbuild.2014.12.038 0378-7788/© 2014 Elsevier B.V. All rights reserved.
legislative motion calling for even tougher more ambitious legislation. The EU Council now needs to reach a position on this strand of proposals. In the meantime, the tertiary and housing sector remains the leading energy consumer in France, at 69.04 Mtoe. Percent of market demand is stable at 43%, but the absolute figures jumped +25% between 1973 and 2008 [1]. Every day, an average EU citizen uses around 35 L of domestic hot water heated to 60 ◦ C, and the trend is an upward curve, as the energy needed to produce domestic hot water has crept up over the years as people have progressively got used to greater indoor comfort. Heating (at 50%) and water heating (25%) are clearly the biggest subsectors in terms of energy consumption (Fig. 1). For lowenergy buildings, water heating and space heating account for 50% and 30% of energy consumption, respectively, with the remaining 20% down to electrical home appliances [2]. Using renewable energies can both improve energy efficiency and reduce fuel or electricity consumption. The chosen solution must be able to fit new or recent housing and, more importantly still, to retrofit old housing stock, which the biggest energy consumer. Solar collectors are a particularly well adapted option for use in this sector. A solar power system can efficiently cover up to 80% of household domestic hot water needs – and all with the added advantages of less fuel used, minimal pollution and little maintenance effort. The thermal solar market is booming, with 2008 figures pointing to a 51.5% jump on 2007 and over 4.5 Mm2 installed
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Electrical auxiliaries, 25% Heang Heang, 50%
Water Heang Electrical auxiliaries
Water Heang, 25%
Fig. 1. Breakdown of housing-sector energy consumption in France.
(3172 MWth) [3]. There has been a fresh surge in research into thermal solar, which is explained by a combination of factors: • Increased energy use tied to producing domestic hot water. • Development of sustainable new solutions for cutting building energy use. • A booming thermal solar market. Introducing innovative and environmentally friendly solutions is a vital task made complicated by the number of bottlenecks involved – some financial, others technical, psychological, and even legislative (building code). The solutions proposed need to be seamlessly building-integratable and minimize eyesore (psychological bottleneck). They also need to be easily installed onto new building and old housing stock (technical bottleneck), not too expensive (financial bottleneck), and environmentally friendly. Our basic idea, in a nutshell, is to ‘activize’ passive components of a build. 2. Thermal solar collectors and the governing French legal landscape From the outset, installations harnessing solar energy have been tarnished by a reputation for creating an eyesore, which has turned many potential users away. On the aesthetics front, photovoltaic systems already offer households and designer large room for manoeuvre, as the cells are small and modular and the electrical cabling is far more flexible to work with than regular pipework, which substantially increases their potential integratability [4,5]. Outside of the sociological dimension – which remains a complex focus of study – there are legislative roadblocks based on heritage asset conservation policy, typically: • Heritage protection perimeters surrounding a historic-interest monument; • Conservation-area districts; • ZPPAUP – conservation areas for ‘Protected Urban, Architectural and Landscape Heritage’. Article 28 of French act no. 2010-788 dated 12 July 2010 was reformed to replace ZPPAUP with AVAP – areas of heritage value; • Listed or scheduled heritage sites. An AVAP project is purposed with the development and enhancement of built-environment heritage and historic spaces, as an extension of committed sustainable development policy. Any construction or development project inside these areas must first apply for planning permission from the STAP – the local-region buildings, monuments and heritage assets authority. An ABF – Architecte des Bâtiments de France, the appointed advisor charged with supervising that any new build or development work bordering onto protected monuments will not create an eyesore – then
issues their local authority with a straight consent, development consent needed or full consent procedure [6]. Unfortunately, solar power facilities do not qualify as “showcasing heritage value”. Furthermore, the ABF also have an influence that stretches beyond AVAPs, as their scope of authority extends to: • The surroundings in the vicinity of historic monuments and listed heritage sites: a protected-area perimeter spanning the 500 m radius around any historic monument [7,8]. This authority was enacted by the French Planning Act of 1943, which stipulates that “any home located in line of sight of a historic monument may not be extended, demolished, cleared of trees or altered in any way that might affect its character” [7]; • Scheduled sites: sites governed by the French Ministry for Sustainable Development, which missions the ABF out to the ground [8]; • Conservation-area districts: the ABF supervises that any first-fix (building envelope) or second-fix (structural interior) remodelling is compliant with the preservation and rehabilitation area map (Malraux Act of 1962 safeguarding historic-interest town centres); • AVAPs: the ABF coordinates the study then checks the projects are code-and-regulations-compliant. Taken together, these areas add up to represent huge potential for working thermal solar energy. As of 31 December 2012, France counted 43,180 monuments, i.e. 14,367 ‘listed’ and 28,813 ‘scheduled’, registered as holding historic interest [9], and the list is getting longer every year, plus the hundred-odd conservation-area districts that cover an average surface area of 6600 ha each [9]. That said, there are signs the legislature is loosening up: • Under round 1 of the multi-party ‘Grenelle Environment’ reform, the French Senate and National Assembly, initially divided on the bill, finally agreed to a measure voted in Thursday 23 July 2009, passed by a special joint government – senate committee. Under this measure, ABFs saw their powers clipped back, as the duty to secure ‘permitted development’ consent was replaced by a straightforward ‘consultative’ ruling in the vast majority of cases. Today, the decision taken essentially follows local zoning code, called PLU – Plan Local d’Urbanisme. In practice, to determine the impact of this kind of measure, it would be necessary to pore through every single ‘PLU’ zoning map of France, as each one is defined at local council level (or, in rare cases, at cross-localcouncil level); • The French Act no. 2010-788 dated 12 July 2010, dubbed ‘Grenelle Environment Round 2’, amended the French Urban Planning Code by ushering in article L.111-6-2, under which paragraph 1 reads: “despite any town planning provisions to the contrary, planning permission may no longer oppose the installation of equipment that produces renewable energy to cover the household energy needs of the occupants of all or part of the building”. Table 1 pools and recaps the various administrative procedures governing project sites according to area location. This short analysis highlights how the stiff French homes and housing legislation privileges aesthetics over energy performance. Although there are signs the legislation is softening up, the conditions imposed are still strict and, in tandem with the financial grants and incentive schemes available, tend to support a policy of maximal integration of energy supply components. Indeed, government directives further anchor this policy priority given to building-integrated solar technologies to foster more landscape/architecture-conscious aesthetics and to position home solar trades and industries on a higher-value-added subsector [11]. Solar energy is coming to town and shaking up the old building
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Table 1 Recap of the procedures applicable to residential build projects according to area location [10]. In heritage protection perimeter surrounding a historic-interest monument
In a conservation-area district
ZPPAUPa /AVAPb
In a scheduled heritage site
In a listed heritage site
- Straight consent when the building is in line of sight of a historic monument - Development consent needed in line of sight of a historic monument
- Development consent needed
- Full consent procedure
- Straight consent - Development consent needed for permission to demolish
- Straight consent with consultative input from the local-region nature, sites and landscapes commission
a b
ZPPAUP – conservation areas for protected urban, architectural and landscape heritage. AVAP – areas of heritage value.
Fig. 2. Illustration of the H2OSS concept installed in situ.
function and design codes. Under this new paradigm, solar power facilities are now energy-producing features of the building envelope. French Thermal regulation 2012 accelerates the process of solar installations acquiring as can do for French tropical island (Reunion) the approach of global quality standards for natural and low energy cooling in buildings [12–14]. 3. Outline of the new solar collector concept The H2OSS flat-plate thermal solar collector concept, as developed and patented [15], was devised as a solution to tackle the above-cited bottlenecks via a strategy based on activizing housing components that have traditionally remained passive. The solar
roof ledge was designed concomitantly with our lab-developed airbased solar shutter system which is equipped with a hybrid PV – thermal solar collector [16] for home space heating. Developed to the fully building-integrated design brief, the H2OSS thermal solar collector is a flat-plate collector designed as a forced-circulation home solar water-heating system. The innovation lies in its long slim geometry enabling it to slot into a pre-profiled aluminium gutter channel used for rainfall evacuation. This aluminium gutter channel profile can be built in place, in one piece (running up to 20-odd metres long), using the industrial extrusion process. The gutter channel obviously retains its rainfall evacuation role. The gutter channelling comes in an array of styles and colourways
Fig. 3. The system installed at the Vignola Lab (Corsica) – overhead view and cross-section view of the H2OSS solar collector.
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Fig. 4. System of fixtures fastening the modules inside the gutter channel.
for enhanced building integration. These modules are quick and easy to install, and the installed system is invisible from ground level, as the main circuit inflow and outflow tubing is hidden away inside the rainfall evacuation tubes (Fig. 2). The H2OSS solar module draws on the most tried-and-tested technology for this type of collector, i.e. a semi-transparent glazed cover topsheet, a highly selective absorber plate, a heat-insulating backsheet, and two copper tubes that recover the heat energy converted by the absorber (Fig. 3). An installation is comprised of a set of modules interconnected in series and/or parallel circuits. Each module measures 1 m long by 0.14 m wide and is connected to the next by a split-point junction making it possible to swap a given module in or out, as necessary. The tube located just under the fin is the outflow tube, and the other is the inflow tube containing the coolant that recovers heat from the absorber. Fig. 4 shows the space that needs to be left between module and gutter channel in order to evacuate rainwater. Note also that the absorber is edged with a pair of folds called ‘fins’. The job of these fins is to increase the captation surface of the otherwise relatively small modules, and trials with prototypes have validated the role of these ‘fins’. The collectors are very easy to fit: • There are no leaktightness issues to deal with, as the collector is no longer in-roof; • The solar system installer no longer needs roofer training – plumbing skills are all that is needed; • The installation is a modular design making for convenient transport and handling; • The modules are fastened onto the gutter channel via fixtures that are invisible from ground-level (Fig. 4). Module tilt can be adjusted to fit each installation configuration; • The inside workings of the installation complete with tubing and storage tank is identical to the current generation of home solar water-heating systems.
This collector is designed preferably to be mounted south-facing but can, depending on site configuration and roof pitch, be installed on north-facing buildings. There is no particular servicing or maintenance required, other than greater attention if the equipment installed is surrounded by deciduous trees. The gutter channels may need clearing or cleaning once or twice a year. The collector itself will suffer relatively little from dust or dirt build-up, and the occasional rain shower is generally enough to keep the system performing to within a few percentage points of peak transmittance [17].
4. Concept characterization, model and validation 4.1. Defining parameters of the solar collector We previously tested the solar collector’s thermal behaviour to validate the thermal model [18] and attempt to improve performances [19] by trialling certain parameter adjustments (Fig. 5). Briefly, the trials were led on a site at our laboratory in the Gulf of Ajaccio (latitude: 41◦ 55 N; longitude: 8◦ 55 E) at an altitude of 70 m asl and about 200 m from the seafront. This trial setup enabled us to operate closer to European Standard EN 12975-1 [20], with four rows of four modules (2.24 m2 ) connected in series and fixed on a solar tracker for better control of solar intensity and direction. The solar modules are connected to a thermal loop that regulates the input fluid temperature by heating the fluid if it is too cold or cooling it via an air cooler if the fluid is too hot. Measurements were taken on each module at 60 s intervals to record: solar irradiance on the collector plane (using a Kipp & Zonen CM11 pyranometer), ambient temperature, humidity, wind speed and direction, fluid flow rate and input and output fluid temperatures. This experimental trial enabled us to plot the performances of the new solar collector. Performance efficiencies were calculated experimentally for various measured reduced temperatures, and linear regression was applied to determine optical efficiency and thermal losses according to Eq. (1): = −KTr + B
with Tr =
(Tm − Tamb ) I0
(1)
where I0 is solar irradiance, Tm is mean temperature, Tamb is ambient temperature, B is optical efficiency (dimensionless) and K represents the thermal losses (W m−2 K−1 ) [21].
Fig. 5. Experimental setup: solar tracker fitted with H2OSS modules.
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Fig. 6. H2OSS® solar collector design as electric circuit analogy.
Table 2 Optical efficiency and thermal losses. H2OSS Optical efficiency, B Thermal losses, K (W m−2 K−1 )
0.903 13.80
Table 2 reports the optical efficiency and thermal losses. While our optical efficiency was high, the coefficient relative to thermal losses was bad. This difference is due to the geometry of the H2OSS® modules, as thermal losses are heavier on the sides of the modules and so performances decrease rapidly when reduced temperature increases. The H2OSS collector performed better with a low reduced temperature and worked better when input water temperature was low, which prompted us to use a water storage tank with thermal stratification making the system work at low flow rate so that the water coming from the tank to the solar collector will be colder.
1 2
3 Fig. 7. Close-up of the first node of the glazed flat plate.
+
4.2. Thermal model 4.2.1. H2OSS numerical model We developed a two-dimensional thermal network model [18] based on nodal discretization method. The solar collector is broken down into 97 finite-element volumes that are assumed to be isothermal, and for each node, we describe the governing thermal equilibrium equation based on an electric circuit analogy (Fig. 6) in which temperatures, sources of flow and boundary temperatures are assimilated to electric potentials, currents, current generators and voltage generators. The thermophysical properties of the component collector materials are assumed to be constant within the effective range of working temperatures. The input parameters used in the model are solar irradiance I0 , ambient temperature Tamb , airspeed over the front face of collector V, temperature at ground level Tground , temperature overhead Tsky , and temperature of the coolant fluid at the collector in-feed. To illustrate, the equation corresponding to the first node of the glazed flat plate (Fig. 7) is detailed in Eq. (2). glass · Cpglass · V4 ·
dT4 4 = S4 · ˛glass I0 −εglass · S4 · f4−sky · (T44 −Tsky ) dt
2
+
Tamb − T4 T3 − T4 T5 − T4 + + Rcond + Rconv Rcond Rcond
n
εglass · Strans2 · f2−n · (T14 − Tn4 )
0
+
T20 − T4 Rcond + Rconv
(2)
where S is surface, ˛ is attenuation coefficient, I0 is solar irradiance, ε is coefficient of emissivity, f is form factor, T is temperature, and R-value is thermal resistance and n multiple reflections in the air layer. All three types of heat transfer are mapped as thermal resistances. Higher R-values mean lower heat flux values. Rcond corresponds to a conductive resistance that is dependent on thermal conductivity of the material, thickness e to be crossed, and heat transfer area S. Rconv corresponds to a convective resistance that is dependent on heat transfer coefficient h. Radiative transfer was expressed using the linearizing Stefan–Boltzmann equation that can translate radiative transfers into thermal resistances. However, for sharper accuracy, we used the non-linear Stefan–Boltzmann equation, which means thermal equilibrium is expressed as stated in Eq. (2).
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For the equation governing energy balance of the fluid circulating inside the copper tubes, the temperature at the coolant tube wall was estimated using the NTU method (Eq. (3)) that produces an estimate of the temperature profile of a fluid circulating inside a homogeneous tube (Ta ) for an internal surface area Sfc under steady-state flow [22]. Tfc/s = Ta + (Tfc/e − Ta ) e−NTU
(3)
where NTU is number of transfer units (Eq. (4)). NTU =
hfc/a Sfc ˙ fc Cpfc m
(4)
Using Eqs. (3) and (4), and knowing the inflow temperature of the fluid and the mean tube temperature, it becomes possible to calculate fluid temperature at the collector outflow. The 97 equations – one for each of the 97 finite-element volumes – were solved numerically. The model thus obtained is resolved using an implicit direct integration method. 4.2.2. The hydraulic loop During the experimental phase, we noted that one of the key problems with our solar collector is hydraulic resistance due to the collector’s linear structure. Consequently, it would be wise to keep this solar system working in low flow-rate conditions. By reducing hydraulic losses, the low rate regime also brings other advantages, as reported in [23]. Thermal stratification: using low-flow operation results in an increased solar collector outlet temperature and consequently induces a higher degree of thermal stratification inside the tank Moreover, water temperature at the top of the tank will be closer to setpoint load temperature, which means auxiliary energy consumption will be decreased, thus increasing solar fraction. Furthermore, with highly stratified heat storage, the return temperature to the solar collector will be lower and the working periods for the solar collector will be longer, which should lead to increased net energy output from the solar collector [24,25]. - Piping in the solar collector loop: low-flow systems can make use of smaller pipes, which means less material needed for pipes and insulation thus less heat losses. - Pump: the energy demand of the circulation pump is decreased, which is a critical factor in our case. The thermal loop behaviour is simulated using a numerical code based on a nodal approach [23–27]. It is divided in 19 nodes: 7 for fluid circulation, 10 for the storage tank (optimal number of nodes to optimally take into account the thermal stratification [23]), and 2 for the water storage inlet and outlet (see Fig. 8). The temperature of the water at the outlet of the solar collector and the average temperature of the solar absorber are obtained by modelling the solar collector, keeping in mind that all 97 temperatures in the solar collector need to be computed to determine these two temperatures. The energy balance, in 1D, is applied and an iterative method is used to solve the first-order differential equations. A reversionelimination mixing algorithm based on a thermal mix of certain storage tank nodes to obtain a correction factor giving a positive temperature gradient from bottom to top of the tank [28,29] was used to simulate in-tank thermal stratification. The tubes between the tank and the solar collector are 9 m long, with 3 m inside the building (ambient temperature is thus the temperature inside the building – thermal losses are thus taken into account). The coil heat exchanger is modelled by five nodes, and thermal power between the heating fluid and the water into the tank is calculated using the NTU method [30].
Fig. 8. Model node definitions [6].
Table 3 Accuracy of the thermal model for the nine temperature points inside the solar module.
RMSE (◦ C) RRMSE (%)
T1
T2
T3
T4
T5
T6
T7
T8
T9
1.65 9.4
1.73 6.4
1.69 4.6
2.36 8.3
2.05 6.1
1.98 5.8
1.39 9.4
1.21 7.9
1.09 8.2
4.3. Experimental validation With implementation of the thermal model, we moved on to validate the model based on experimental data collected with the second experiment (including a solar tracker) [18]. A thermal solar module was specially instrumented with nine thermal sensors (type PT100 class B) measuring surface temperature at nine specific points (Fig. 9): (1) On the glass; (2) on the right fin; (3) on the left fin; (4) on the right absorber; (5) on the left absorber; (6) in the right insulation; (7) in the left insulation; (8) on the right side and (9) on the rear face. The solar thermal modules are connected in parallel delivering a water flow rate of 450 L h−1 ; in these conditions, the increase in water temperature is small but the temperature gradient inside the module is more clearly visible. An experimental verification was carried out on the all-year data. The setup demonstrated good accuracy between modelled and experimental temperatures. We calculated the absolute and relative root mean square errors (RMSE and RRMSE) over the total end-to-end experimental period and over four seasons (about 100 days). RMSE and RRMSE values for the nine temperatures calculated from Eqs. (5) and (6) are given in Table 3.
RMSE =
n (X i=1 sim,i
RRMSE =
− Xexp,i )2
(5)
n
n [(Xsim,i i=1
− Xexp,i )/Xsim,i ] n
2
(6)
where n is number of data and Xsim,i and Xexp,i are respectively simulated and experimental data. The simulated and experimental temperatures show fairly good fit. We also simulated output water temperature with the thermal module connected in series and delivering a water flow rate of 140 L h−1 . These conditions were chosen to approach real operating conditions. We computed RMSE and RRMSE for output water temperature over four seasons (using about 90 days) and obtained the following values: RMSE = 1.4 ◦ C and RRMSE = 5.2%. These errors are fully acceptable. These two validations certify that the model is
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Fig. 9. (a) Temperature sensors positions, (b) temperature sensor, (c) and (d) instrumented thermal modules.
Table 4 Optical efficiency and thermal losses.
solar system in order to benchmark the energy performances of the installation. The installation set up comprises a 200 L electric–solar hot water storage tank with a 2 kW resistance. It features 20 serially connected H2OSS solar collectors covering a total working surface of 2.8 m2 . The main solar circuit comprises 9 linear metres of pipework between first collector and storage tank, including 6 linear metres that are outside the collector system loop. The installation runs at 80 L h−1 m−2 . Table 5 lists the recorded mean monthly demand-water temperatures and mean monthly outdoor temperatures. Fig. 11 charts the year-averaged over-day demand profile recorded for this detached home housing four occupants (two parents and two children).
H2OSS Optical efficiency, B Thermal losses, K (W m−2 K−1 )
0.780 6.25
robust to be used in future work studying the influence of changes in materials, structure, or other possible modifications. 5. Process of evolving this solar collector into an optimized version The solar collector was numerically optimized for a conventional installation in single-family housing in the south of France. We previously showed [19] that repositioning the cold water tube into the absorber rather than the insulation layer yields much better performances than with the current prototype. Thermal insulation and air layer thicknesses were then optimized, and the influence of water flow rate proved very high by virtue of the design of this new solar collector. Three configurations were studied (Fig. 10). The configuration that best optimizes performances is configuration ‘b’ with the two parallel tubes on the same plane. Table 4 gives the new computed optical efficiency and thermal losses of configuration ‘b’. This new configuration of the H2OSS concept will be simulated for a single-family house, and performances will be compared against the installed prototype.
6.1. Results of the installed running prototype We measured energy performances over a 12-month year (2013) in operation. This solar power installation delivers a solar fraction of 27%. Over the year 2013, we get a net solar power output of 835 kWh for a domestic hot water demand of 3055 kWh and a solar radiation of 4901 kWh received. Fig. 12 charts solar radiation received, domestic hot water demands, effective solar power output and solar fraction monthby-month over 2013. Note that over the trial year 2013, the installation showed energy losses on the main solar circuit (tubing) amounting to 93 kWh and losses via the electric–solar hot water storage tank amounting to 371 kWh. Annual energy consumption of the solar circuit pump amounts to 63 kWh.
6. Real-world performances of a solar facility located in southeastern France (Bastia, Corsica; Fig. 2) and simulated performances computed for this new concept
6.2. Simulated performances computed for a new-version H2OSS We simulated energy performances over a 12-month year (2013) in operation for the optimized new-version H2OSS solar collector. This solar power installation now delivers a solar fraction of 48%. Over the year 2013, we get a net solar power output
This building-integrated solar thermal system is located in Corsica, at latitude 42◦ 42 10 N, longitude 9◦ 26 59 E, altitude 51 m asl, in a Mediterranean climate zone. We instrumented the thermal
Table 5 Recorded mean monthly demand-water temperatures and mean monthly outdoor temperatures.
Demand-water temperatures Outdoor temperatures
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
8.6 9.2
8.2 9.5
8.8 11.9
10.5 13.5
12.6 18.9
14.7 22.8
15.9 24.9
16.6 25.8
15.8 20.8
14.2 17.6
12.0 12.8
10.5 10.0
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Fig. 10. Different studied configurations.
600 500 Solar Radiaon (kWh) 400 Domesc hot water demand (kWh) Effecve solar producon (kWh) Solar Fracon %
300 200 100 0
Fig. 13. Energy performance (K = 6.25/B = 0.78).
Fig. 11. Average daily profile for domestic hot water supply.
the
simulated
solar
thermal
system
7. Conclusion
600 500 Solar Radiaon (kWh) 400 Domesc hot water demand (kWh) Effecve solar producon (kWh) Solar Fracon %
300 200 100 0
Fig. 12. Energy performance (K = 13.80/B = 0.903).
of
of
the
existing
solar
thermal
system
of 1408 kWh for a domestic hot water demand of 3055 kWh and a solar radiation of 4901 kWh received. Fig. 13 charts solar radiation received, domestic hot water demands, simulated solar power output and solar fraction monthby-month over the 2013. This new version, as simulated over the trial year 2013, showed energy losses on the main solar circuit (tubing) amounting to 121 kWh and losses via the electric–solar hot water storage tank amounting to 429 kWh. Annual energy consumption of the solar circuit pump amounts to 64 kWh.
Despite extensive deployment of solar power worldwide, the solar economy still faces a number of bottlenecks. One of the takehome messages from an R&D seminar hosted by the ADEME – French Environment and Energy Management Agency at Sophia Antipolis (France) on 27 and 28 April 2004 to address “innovation needs for thermal solar power” was that the thermal solar systems on the market are essentially unintegrated “add-ons” to the building. The problems that emerged – both on the technical front and the aesthetics front – are patent bottlenecks to development. Furthermore, as the solar market has now gained relative maturity, solar product costs have begun to stabilize and are unlikely to show any fresh drop in the near future. The upshot is that short of any economies of scale driven by a boom in the market, only a disruptive technology causing a shift in design-engineering could spark a significant new shift in solar economics. The bottlenecks, in order of importance, are first financial, then technical, legislative, and finally psychological. It is in this context that we propose a new home-integrated solar power concept. This paper outlines the French legislative constraints governing the deployment of thermal solar collectors in France, before going on to introduce the new integrated concept by outlining the modelling process and model validation ready for use in simulations. We report a running prototype installed in Corsica, France, and its energy efficiency performances achieved over the year 2013. This solar power installation delivers a solar fraction of 27% for a net solar power output of 835 kWh. Simulations of a new optimized version of the solar collector prototype run for this same installation yielded improved results, with a solar fraction of 48% for a net solar power output of 1408 kWh – and consequently a net gain approaching 78%. This research addresses EU energy policy challenges tied to integrating
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