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mproving the renewable energy mix in a building toward the nearlyero energy status
on Visa, Macedon D. Moldovan ∗, Mihai Comsit, Anca Dutaransilvania University of Brasov, Renewable Energy Systems & Recycling R&D Centre, Bd. Eroilor 29, 500036 Brasov, Romania
r t i c l e i n f o
rticle history:eceived 1 August 2013eceived in revised form 22 August 2013ccepted 16 September 2013
eywords:early Zero Energy Buildingnergy mix
a b s t r a c t
Developing Nearly Zero Energy Buildings (NyZEB) represents a path toward sustainable communities andis required by international regulations, starting with 2018. Combined measures for reducing the energydemand and increasing the share of renewable energy systems in buildings are very much investigated fordifferent types of buildings. One specific case is represented by the buildings where – as result of the greenenergy policies – renewables are already installed, but the NyZEB status is not reached yet. These buildingsare main candidates in getting this status as the initial investment required is significantly lower. A novelmethodology is presented for this type of buildings aiming at identifying the optimal combination of
olar Houseuilding integrated photovoltaics
actions to be taken for reducing the energy demand and developing optimized renewable energy mixes,integrating the existing ones, up to the (Ny)ZEB status. Following this methodology, a cases study ispresented – the Solar House (low energy building with geothermal system and solar energy convertors)and the steps followed for reaching the Zero Energy Building standards are presented. Considering thecurrent energy status of the building, the renewable energy potential and the costs, a tracked PV string
ded
array is proposed to be ad
. Introduction
The Energy Performance of Buildings Directive (EPBD) stateshat buildings account for 40% of total energy consumption in theuropean Union (EU) [1], and similar figures are valid for US (the010 value was 41% [2]) while a significant 25% is reported forhina [3]. Many other studies are devoted to this aspect, result-
ng values of energy consumption in the building sector between0% and 40%, exceeding the industrial and transportation sectors
n developed countries [4]. About 10–20% from the energy con-umption is embodied in the construction stage while 80–90%epresents the energy use operating the building during the lifeycle [5]. The energy demand during the lifetime of the existinguildings is expressed in terms of annual specific primary energyse which falls in the range of 150–400 kWh/m2 per year for con-entional residential buildings and 250–550 kWh/m2 per year forffice buildings [5], values that are very high considering the stan-ards of 60–80 kWh/m2 per year set for low energy buildings.
The energy consumption depends on the climatic conditions,sers’ behavior and on the building as such, consequently on the
n force regulations when the building was constructed whichmposed (or not) a minimum set of performance characteristicsor the building materials. Considering the expected growth of
population, up to 10 billion till 2050 [6], the expansion of the build-ings sector is predictable, along with an increase in the energyconsumption; therefore, for being sustainable, there is a need forsolutions that will support a much lower rate in the energy increaseas compared to the population growth.
The main requirements for the sustainable built environment,as described by the Trias Energetica Concept, are related to lower-ing the energy demand by combining energy saving, the efficientuse of energy and waste minimization with increasing the share ofrenewable energy systems (RES) and using fossil fuels in the clean-est possible way, this also including emissions mitigation. Thus, thesustainable built environment cannot be developed without sus-tainable energy systems and these are intrinsically linked to spatialplanning [7], in order to prevent unbalancing the resources and toavoid the competition with agriculture and forestry.
To speed up the process, legal instruments were launched, asit is the EPBD stating that by 31 December 2020, all new build-ings should be Nearly Zero Energy Buildings, with implementedcost-effective solutions both in energy efficiency measures and inenergy-supply systems including renewable energy sources [1]; asresult plenty of work was devoted in the past five years for iden-tifying adequate and customized solutions promoting low energyconsumption and building integrated renewables [8–10], partic-
ularly focusing on the new buildings. This leaves the existentbuildings as large energy consumers; being developed mostly inthe ‘70s or before, these buildings are still functional but need(at least) refurbishing for lowering their energy need, therefore a
pecial target is set through EPBD firstly for public buildings. Thiss important especially because new buildings only account for aery low percentage in the residential building stock (in EU 1.94%11]).
Attempts for promoting energy efficiency in buildings wereone but the pace is much slower than initially planned and the rea-ons are mainly related to the acceptance coming from the buildingwners, to the conservative building industry with a huge num-er of suppliers, builders, designers and developers with lack oforizontal integration [12], and finally to the costs issues, raised
rom both parts. Therefore, sustainable and affordable solutionsre needed, making use of the available infrastructure and energyesources based on an integrated design. This is important becauseenerally, through integrated design up to 90% reductions in thenergy use can be achieved in the new low-energy buildings andore than 50% for the existing building renovation, almost every-here in the world [13].
A specific category is represented by the buildings where differ-nt renewable energy systems were installed in the past years, asesult of the support policies promoting sustainable energy. Being
direct (and fast) result of an opportunity, the systems in theseuildings were separately designed and are separately operated,heir mission being to reduce the consumption based on tradi-ional (fossil) energy sources. As already having part of the requirednfrastructure, the conversion of these buildings toward the Nearlyero Energy Building status involves lower investments and makehem priority candidates for implementing the legal frame and foreveloping sustainable communities. To fully valorize the potentialf these buildings, a specific integrated design is required, combin-ng measures for reducing the energy demand and increasing theenewables.
This paper proposes for the first time (to the best of ournowledge) a design method mainly dedicated to these buildings,llowing cost-effective solutions. Following this method the pathor reaching the Nearly Zero Energy Building (NyZEB) status isescribed based on in-field data, for The Solar House – a build-
ng that has implemented renewables and will be transformed inyZEB.
. Methodology
The methodology for reaching the NyZEB status in buildingsith already installed RES follows three steps that are based on
he evaluation of the existent energy status followed by step-wiseeasures for increasing the energy performance and extend the
ES share up to the targeted level. The schematic description of theethodology is presented as a flowchart in Fig. 1.Step I. Current energy status, aims at defining the input data as
ollows:
Building characteristics considering the geometry, the envelope(materials, thermal and optical characteristics), the building type(household, office or industrial building), the inhabitants, etc.Additionally, spatial limitation for further implementing RESshould be included (e.g. yet available roof, terrace or facades areas,with optimal orientation for solar energy convertors).
Characteristics of the implementation site. These data areincluded in the calculation of the building energy demand andallows estimating the renewable energy potential. This can bedone based on generated data using dedicated software, mainlyrelying on the geographical coordinates (latitude, longitude,
height, clear sky index, etc.); using dedicated software has theadvantage of minimal input information but can lead to under-or over-estimation of the renewable energy potential, with neg-ative consequences on the final energy output and on the costs.
dings 68 (2014) 72–78 73
Therefore, on-site data (collected at least over one year) are rec-ommended for accurate RES design.
- Implemented RES, in terms of type (RES1 . . . RESk, e.g. photo-voltaic, wind, solar thermal systems, heat pumps, biomass, etc.)and energy output (average and extreme values); the total energyoutput of the renewable energy systems, RE, is calculated as thesum of the components (RE = ˙REi, with i = 1 . . . k).
- Standardized indicators for renewable energy systems (initialinvestment cost, exploitation cost, payback time, cost/benefitratio, CO2 emission savings) and for Nearly Zero EnergyBuildings.
The input data are used to calculate the energy demand (ED)of the building, using the building’s characteristics, the weatherdata and the utilization type of the building. The energy demandconsiders the space heating and/or cooling load, the domestic hotwater load and the power load for lighting. Elaborate or simplifiedengineering methods, statistical methods, neural networks, etc. arereported [14], and the major challenge is to correctly identify themodels’ input parameters in order to describe the building energyflow.
Based on the energy demand and on the share of renewablesin the existing energy mix, the need for further implementing RESis evaluated. If the energy demand is fully satisfied by the alreadyimplemented RES the Zero Energy Building (ZEB) status is reached;if RE exceeds ED, the building has a Plus (Green) Energy status (PE)and, as also in the case of ZEB, no further measures are needed. Itis to mention that the ZEB or the PE status can be reached duringcertain seasons in the year (most likely during summer) but themethod hereby described proposes calculations covering one fullyear, as many other authors agree [15].
But, most of the existent buildings have the energy demand notfully covered by RES energy production. Two paths can be followedto meet the NyZEB status: decreasing the energy demand and/orincreasing the share of RES.
Step II. Decreasing the energy demand. It is unanimously agreedthat installing RES is output- and cost-effective in buildings wherepervious energy efficiency and energy saving measures were imple-mented [16]. Thus, reducing ED is the next step by implementingthe first and second stages of Kyoto Pyramid [17], by reducing theheat losses and by ensuring an efficient electricity use. In a practi-cal approach, decreasing ED can be obtained by different measuresthat should be implemented according to the building’s specifics:refurbishing the envelope, efficient lighting and equipment, evenmeasures of passive solar design, if possible.
These combined measures can have a significant influence onthe building performance, therefore a new assessment of the EDvs. RE is done. Similarly to Step I, if the ZEB or the PE status isreached, no further actions are required. If the measures for low-ering the ED are not enough to meet the NyZEB status, Step III isfurther approached.
There might be situations when reducing the energy demandis no longer possible, as common and affordable solutions werealready implemented. In these situations, the procedure furtherruns from Step I directly into Step III.
Involving the beneficiaries in the design process represents a keyissue in increasing the acceptance of the sustainable energy solu-tions. Therefore, the method considers an inquiry step, when theusers receive the quantification of the building performance withlowered ED and are asked if they further want to increase RE. Asthe NyZEB status is not strictly defined, the answer can be “No” (noneed for further renewables), and in this case, no further action is
taken. But, considering the legal frame and the conventional energyincreasing costs, this option is likely to be less and less selected inthe future, most of the users opting for an increase in the energyobtained based on renewables.
74 I. Visa et al. / Energy and Buildings 68 (2014) 72–78
Fig. 1. The algorithm defining the optimal energy mix toward NyZEB (RES – Renewable Energy System, RES1 . . . RESk – already implemented RES, RESk+1 . . . RESn – newlyp le EnR
cRcee
tst
roposed types of RES, ED – Energy Demand of the building, RE1 . . . REk – RenewabES, RE – Renewable Energy provided by all RES).
Step III. Increase the energy from RES considers improving theapacities of the already implemented renewable energy systemsES1 . . . RESk with increments �RE1 . . . �REk that are selectedonsidering the design of cost-effective and environmental friendlynergy mixes; another option is to include new types of renewablenergy systems RESk+1 . . . RESn with increments �REk+1 . . . �REn.
The increments �REi are iteratively considered, in respect withhe energy needs of the building and with the site renewable energyources potential. During each iteration renewable energy sys-ems indicators are evaluated and if they are not acceptable, in
ergy provided by already implemented RES, �REi – increments proposed for each
comparison with standardized indicators, increments �REi arechanged until acceptable values are obtained.
The optimization criteria based on which a certain energy mixis selected usually relay on the payback time, considering a fixedratio of renewables, with a fixed share of each renewable compo-nent in the mix, and optimizing the technical parameters of the
system (e.g. the surface of the solar-thermal array, the ground heatexchanger surface, etc.). There is hundreds of software developedto design energy mixes in buildings. Recent review papers outlinethe mostly used tools; among the 38 (out of 67) software selected
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y Connolly et al. [18] 16 included renewables, while Crawley et al.19] reviewed 20 software for estimating the capabilities of buildingnergy simulation programs out of which 14 included renewables.
These type of tools need to be re-designed by including theavings brought by the existent RES, particularly parts of theirnfrastructure that can be commonly used by the existent andewly implemented renewable energy systems. Additionally, noveloncepts can be added, as the Building Energy Management Sys-ems (BEMS) including RES, in multi-source/multi-function BEMSble to efficiently operate the energy mixes including renewablenergy technologies. The built environment, with restricted imple-entation space can also support solutions that are recognized to
ncrease the energy output, as example tracking for solar energyonversions systems.
After selecting the optimal energy mix, the Renewable EnergyRE) and the Energy Demand (ED) are once again compared and ifhe NyZEB status is reached, the method proposes different evalu-tion indicators: the initial investment cost, the exploitation cost,he cost/benefit ratio, etc. Choosing among these is upon the end-ser options and/or possibilities, each of these providing the energyo reach the Nearly Zero Energy Building Status.
If the NyZEB status is not reached yet, the calculation ofnergy mixes is re-launched, either by extending the existent onesif possible) but mainly by considering new renewable energyources.
This methodology was implemented for a specific application,he Solar House, which is further presented.
. Case study: the Solar House
The methodology previously described was applied for a con-rete case, the Solar House in the Transilvania University of BrasovRomania), the R&D Center Renewable Energy Systems & Recycling.
.1. Step I. Current energy status
Building’s characteristics: The Solar House was built in 2009iming at developing a low energy building with reduced CO2missions, architecturally integrated in the university campus androviding a high level of individual comfort to its inhabitantsmainly M.Sc. students, the house being used for teaching and as 1:1esting rig). The Solar House is integrated in the university Campus,s Fig. 2 presents.
The design of this building followed the Kyoto Pyramid strat-
gy [17], addressing the reduction of the heat loss through theuilding’s envelope, reducing the energy consumption by efficient
ighting with very low energy consumption and embedding ele-ents of passive solar use.
Fig. 2. The Solar House and the existent renewable energy systems.
dings 68 (2014) 72–78 75
The Solar House has two levels with a total floor area of 290 m2,out of which 240 m2 are heated, corresponding to a volume of777 m3 (the rest of the surface is represented by a not heatedstaircase). The house has a light metallic structure covered withinsulated panels outwards and plasterboard inwards; the large,double-glazed windows have two-layers low-E panes with thethermal resistance 0.33 m2K/W while the thermal resistances of theother building elements are: 3.3 m2K/W for the walls (with 20 cm ofcellular polystyrene thermal insulation), 2.2 m2K/W for the ceiling(covered with 10 cm of polyurethane) and 2.5 m2K/W for the floor.The exterior surface of the house is represented by walls (133.5 m2),the ceiling (115 m2) and windows (130 m2).
The passive solar design is supported by the large glazed sur-face (over 50% of the heated floor surface, as compared to the usual10% . . . 15%) that allows maximizing the solar gain during the coldseasons, and give best use to natural lighting. Significant energysavings were also obtained by the natural ventilation insured, bothin heating and cooling, through the non-traditional egg shape archi-tecture [20].
Characteristics of the implementation site: The geographic coordi-nates of the implementation site are 45.65◦N, 25.59◦E at an altitudeof 600 m above the sea level. The climatic profile is continental tem-perate with cold winters (lowest temperatures reaching −28 ◦C)and warm summers, with peak temperatures of 30 . . . 32 ◦C.
Regular energy utilities facilities are available (grid and naturalgas). The on-site renewable energy sources potential was moni-tored since 2006 (Delta-T wheatear station) and shows an averagesolar energy potential of 1000 . . . 1200 kWh/year and a very lowwind potential (below 2 m/s average wind velocity) [21]; geother-mal energy is and can be further taped in the close vicinity of theSolar House while the hydro potential is practically zero consid-ering that the Solar House is situated almost in the center of thetown. Thus, only solar and geothermal energy can be considered asavailable on-site renewable energy sources.
Implemented RES: Considering the available potential, the SolarHouse has installed a renewable energy mix consisting of a horizon-tal closed loop ground source heat exchanger coupled to a 10 kWheat pump (Viessmann) for heating and cooling, six flat plate andthree vacuum tube solar thermal collectors used for domestic hotwater in the Solar House and in the Sports Hall nearby (Viessmann),and a 10 kWp photovoltaic array, fixed and tilt at 48◦ (Q Cells, 48PV modules, 210 Wp) [22].
The energy demand was calculated based on the specific Roma-nian directives [23], for lighting, domestic hot water and forheating (considering during the cold season the conventional out-door temperature of −21 ◦C and the indoor temperature of 20 ◦C).Considering the climatic profile, the insulation and the main func-tion of the building (teaching and R&D activities), cooling was notconsidered necessary, as the hottest period (August) is also a holi-day month.
The values corresponding to the calculated energy demand [20]and to the energy produced by renewables as measured are pre-sented in Table 1.
As the data in Table 1 shows, the energy demand exceeds by9.62 MWh/year the renewable energies output (RE). Currently, thedeficit in heating and DHW is covered by gas (1.97 MWh/year) andthe electricity deficit is covered from the grid (7.65 MWh/year).These results outline that further actions should be taken for reach-ing the (Nearly) Zero Energy status.
The energy consumption of the Solar House is monitored for:(a) the power consumed by the lighting and heat pump systemswith a three phase electric meter, (b) the thermal energy providedby the heat pump system, by the solar thermal collectors and by
the backup source (the natural gas condensing boiler) with dis-tinct heat meters and (c) the natural gas consumption with a gasmeter.
76 I. Visa et al. / Energy and Buildings 68 (2014) 72–78
Table 1Calculated energy demand and measured energy supplied by renewables in the Solar House.
Thermal energy [MWh/year] Electrical energy [MWh/year] Total energy [MWh/year]
Heating Domestic hot water (DHW) Demanda Supplied by RES Demand Supplied by RES
Demand Supplied by RES Demand Supplied by RES
45.51 44.51 1.25 0.28 12.75 5.10 59.51 49.8997.80% RES 22.40% RES 40% RES 83.83% RES
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a In estimating the electrical energy demand the lighting loads were considered (111.5 MWh/year).
The energy provided by the already installed Res is also mon-tored: the power output of the fixed photovoltaic system with ahree phase electric meter, the geothermal energy extracted fromhe ground and the thermal energy from the solar thermal collec-ors with distinct heat meters.
.2. Step II. Decreasing the energy demand
Being initially developed as low energy building, the Solar Housellows little improvement in terms of reducing the Energy Demand.ased on the monitored data, thermal energy losses were evalu-ted for the main components of the building and the results areresented in Fig. 3.
As expected, the major energy losses are through the glazedreas, therefore simulations were done considering the addition of
third glass pane. The results show that triple glazing will improvehe thermal resistance up to 0.44 m2K/W, reducing the thermaleating load to 37 MWh/year. An analysis of the constructive solu-ions showed that the triple glazing should fully replace the existentouble glazed area and, by market investigations it was concludedn an investment of about 23,500 EUR. As this amount is quite high,urther options were investigated in terms of increasing the energybtained from renewables.
.3. Step III. Increasing the energy from RES
The NyZEB or the ZEB status can be achieved for the Solar Housey replacing the existent traditional energy sources (grid or gasased) with renewables; therefore possible alternatives were fur-her investigated.
Decision on the RES mix: based on the renewable energy poten-ial and the implementation restrictions it was further decided toxtend the existent RES. The existing ground coupled heat pumplready covers almost the entire space heating load therefore thextension of the energy mix was considered for covering the most
nbalanced component: electrical energy, by implementing a newV platform.
Extending the RES mix with a PV string array: as Fig. 2 shows,he available area (light gray) can be used for developing a new
Fig. 3. Energy flows through the building components.
Wh/year) along with the energy needed to power the heat pump and its compressor
platform. The terrace is rectangular (39 m × 18.5 m). As the terraceis not fully favorably oriented, the optimal solution is representedby strings with 16 m and 8 m length, that can be well placed with adistance of 4 m between them for avoiding shading, Fig. 4.
To fully cover the energy need (currently covered from the gridand by using natural gas) by using renewables, a maximal energyoutput is needed for this platform. Tracking increases by at least20% the energy output, thus at least 20% of the modules’ surfacecan be reduced. The 10 MWh/year deficit can be covered by a fixedplatform (of about 16 kWp, with an average cost of 24,000 EUR) orby a tracked platform, of about 10 kWp, with an estimated cost of17,000 EUR, including the tracking system. Using the grid as storagealternative also allows to insert the temporary exceeding energy,produced during summer when the consumption is low (holiday)and to balance over the entire year the needs with the renewablesupply. Additionally, replacing gas with green power has also theadvantage of reducing on-site the CO2 emissions. Considering thesedata, the further option was for the tracked PV string platform of10 kWp.
For avoiding too complicated mechanical systems, one singleaxis tracking system is proposed, of pseudo-azimuthal type. The 3Dvirtual model of the photovoltaic tracked strings (developed usingSolidWorks software) is presented in Fig. 4. Further on, throughSolidWorks Simulation the design was validated, by testing howthe photovoltaic platform holds up under extreme wind, heat, andother conditions. Thus, weight was reduced, unneeded materialseliminated, the costs were optimized, and the potential liability orsafety issues were solved.
To cover the energy need, the PV system consists of 50 photo-voltaic modules of 200Wp (polycrystalline silicon, in-field nominalefficiency 12.01%), a 10 kW inverter and the connecting circuitsbetween the PV modules and inverter (DC) and between theinverter and the national power grid (AC). Power will be introducedin the national power grid all year round and will be used whennecessary (especially at night and during the cold season).
The tracking algorithm that insures the increased electricalenergy output needs to be carefully optimized.
Literature mentions two different types of tracking control: by
pre-set (usual stepwise) algorithms or by following the maximumsolar radiation on the sky dome. As the implementation site has achanging climate, with fast alternating periods of cloudiness and
Fig. 4. The 3D virtual model of the photovoltaic tracked platform and its placementon a rooftop.
I. Visa et al. / Energy and Buildings 68 (2014) 72–78 77
Table 2Calculated energy demand and estimated energy supplied by renewables in the Solar House after implementing the new PV string array.
Thermal energy [MWh/year] Electrical energy [MWh/year] Total energy [MWh/year]
Heating Domestic hot water (DHW) Demand Supplied by RES Demand Supplied by RES
Demand Supplied by RES Demand Supplied by RES
12.75 11.13 59.51 57.8987.29% RES 97.28% RES
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Fig. 6. Electrical power for the tracked and fixed modules in a typical sunny day(the dashed area represents the supplementary energy output due to tracking).
45.51 45.51 1.25 1.25
100% RES 100% RES
unshine, the last option is not recommended because it may leado mechanical shocks and fast wearing.
A stepwise algorithm was developed covering one year,onsisting of 26 intervals (each of two weeks) with different con-gurations, ranging from 8 to 12 daily steps. This algorithm waseveloped in the clear sky assumption. Still, cloudiness may sig-ificantly distort the tracking effect, therefore in-field experimentsere done for defining the optimized version.
As the tracking effect is almost similar on a single module and on module part of a string, the optimization studies were done usingne fixed module and one tracked module, following the proposedlgorithm, both installed on the rooftop, in close vicinity to theolar House. Identical PV modules were used having Pmax = 130 W,
mp = 7.5 A, Ump = 17.3 V, nominal efficiency 13.1%.The input data (global and diffuse solar radiation intensity on a
orizontal plane, air temperature, wind speed and direction) andhe output data (voltage, current intensity and power generatedy each photovoltaic module and for each battery) are monitorednd stored and were processed with common software (Matlab,icrosoft Excel etc.). As example, Fig. 5 presents for the month
f June 2012 the variation of the daily electric energy generatedy the two PV modules (tracked and fixed). The average conver-ion efficiencies were 8.24% for the tracked module and 6.24% forhe fixed module, significantly lower than the nominal efficiency,s result of a combined effect of heating and highly variable dailyolar radiation (200 . . . 1100 Wh/m2). The data in Fig. 5 also allowalculating the energy amount produced by the tracked module2,856 Wh/month which is by 28% higher than the value corre-ponding to the fixed module.
As already outlined, the in-field weather data can be signif-cantly different as compared to those generated by dedicatedoftware (e.g. METEONORM). This may have significant effectn tracked PV systems. Therefore, two representative days wereelected, a typical sunny day (July 1st, 2012) and a cloudy day (28thay, 2012). Based on the voltage and current values for the two
ystems (fixed and tracked), the instantaneous power of each pho-ovoltaic module is further calculated and, by integration over thentire day, the amounts of produced energy are evaluated and areresented in Figs. 6 and 7.
As expected, the solar radiation input has a major influence onhe power output. Photovoltaics are mainly using the direct solar
adiation (usually representing over 70% from the global radiationn clear sky conditions) but, as Fig. 7 shows, diffuse radiation (thenly available during cloudy periods) also allows PVs to function,
ig. 5. Comparative electrical energy output of a tracked and of a horizontal fixedodule.
Fig. 7. Electrical power for the tracked and fixed modules in a typical cloudy day.
with a much lower output, as a combined result of the lower inputand decreased efficiency.
In a sunny day, the energy output of the tracked module(946 Wh/day) is significantly higher compared to the fixed one(711 Wh/day), particularly during morning and afternoon. At noon(11:00–14:00) the photovoltaic responses of the tracked and fixedsystems are almost identical, indicating that tracking is actu-ally not needed. During cloudy days, the results show that thefixed horizontal photovoltaic module has a better energy response(166 Wh/day) than the tracked one (148 Wh/day). This may beexplained considering that the fixed system receives solar diffuseenergy during the entire day from the entire sky dome when hori-zontally positioned, while the mobile system reaches this conditiononly at noon when its position is horizontal. These data indicate thatin cloudy days tracking is not recommended. Therefore, specificactions are implemented in the tracking algorithms for days withlow solar radiation, for getting the optimal photovoltaic response;additionally, this will reduce the overall power consumed for track-ing, reduces the wear of the actuators and of the elements in relativemotion and improves the dynamic resistance to wind (with usualhigher values in cloudy days).
Based on these data, an average amount of energy ranging from8 to 8.5 MWh/year is expected, that covers most of the energydemand, making the Solar House a Nearly Zero Energy Building,with less than 3% of the Energy Demand covered by grid (Table 2).
4. Conclusions
The paper introduces a novel method for transforming build-ings with implemented renewable energy systems in Nearly ZeroEnergy Buildings. The method consists of three steps that sequen-tially evaluate the current energy status, identify tailored measures
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or reducing the energy demand, followed by the development ofn-site optimized renewable energy mixes, by extending the exist-nt ones and/or by adding new renewables.
The Solar House is discussed as a case study, outlining the deci-ions that can be done, considering both efficiency and costs. Bypplying the proposed methodology, the Solar House reaches theyZEB status by extending the existent renewables mix (solar andeothermal) with a photovoltaic string platform that has a trackinglgorithm optimized considering the climatic profile.
This method can be applied to any building that alreadyas installed renewables, by identifying the possible alternatives,ptimizing them according to the on-site data and select-ng the appropriate one, based on technical and economicriteria.
cknowledgements
This paper is supported by the Sectoral Operational Programuman Resources Development (SOP HRD), financed from theuropean Social Fund and by the Romanian Government underhe contract number POSDRU/89/1.5/S/59323 and by the projectroject EST IN URBA, No. 28/2012, PN-II-PT-PCCA-2011-3.2-1235,eveloped within the program PNII – Partnership in priorityomain, with the support of ANCS, CNDI-UEFISCDI.
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