Accepted on 1 May 2015
Submitted on 13 Dec 2014
Whatever the building to build or manage, solutions to control energy consumption must be sought. This is true in the world for all types of buildings, industrial, commercial or residential. Before designing or improving a building, it is essential to study its energy needs and energy sources available, and then look for the best adequacy of management systems, distribution networks and consumer equipment taking into account operating requirements.
The power industry emissions were 10.9 giga-tonnes of carbon dioxide equivalents (GtCO2e) per year in 2005, i.e. 24 % of global Greenhouse Gas (GHG) emissions, and this is expected to increase to 18.7 GtCO2e per year in 2030 (Jelle et al. 2012). The world population is estimated at 8.2 billion people for an energy consumption of15.3 billion-tone oil equivalent (toe) in 2030 (International Energy Agency (IEA) 2012). There is a statistically relation between population and economic growth, energy use, and CO2 emissions (Mohd Shahidan et al. 2013). The population growth increase energy use. An important part of this energy is use in the building sector, that represent about 40-50 % of the energy consumed in developing countries and will be responsible nearly 4300 toe of CO2 emissions for these country in 2030 (International Energy Agency (IEA) 2012). In Cameroon, the energy demand still remains unsatisfied and the access to modern energy is very low, in the national average range of 15 % for electricity and 18 % for domestic gas. In addition, the electricity access is less than 5 % in rural areas against 50 % in urban areas. All sectors combined, the Cameroonian final energy consumption amounts to approximately 5235 Ktoe in 2006 (SIE-Cam; AES-Sonel; CSPH, Nkutchet, 04).
Energy is used for the building comfort that means cooling in tropical region. The construction of low energy buildings is an effective solution that achieves significant energy savings. Low-energy buildings use passive techniques, such as optimal solar gain, and advanced active systems, such as mechanical ventilation with heat recovery, to create comfortable internal environments that have low energy demand. Renewable heating systems including biomass boilers, active solar water heating and ground source heat pumps can be used to supply heating and hot water needs with reduced gas emissions. Solar photovoltaic can be used to provide electricity. Solar energy systems can play an important role in reducing building energy consumption (Hestnes 1999) in tropical region because of it abundance.
Building integrated photovoltaic (BIPV): The concept where the photovoltaic element assumes the function of power generation and the role of the covering component element has significant influence on the heat transfer through the building envelope. Kimura (Kimura 1994), Taleb and Pitts (Taleb & Pitts 2009), and (Zhai et al. 2008) have illustrated various methods of installing the PV modules into a building for a concept of green building. In modern buildings, windows play an important role in energy performance with respect to heating/cooling loads and artificial lighting requirements. The relationship between window design and building energy performance has been extensively researched (Stegou-Sagia et al. 2007; Iqbal & Al-Homoud 2007; Lee & Selkowitz 2006; Wong et al. 2005; Bodart & De Herde 2002; Zain-ahmed et al. 2002; Mehlika et al. 2000; Al-Homoud 1997). Ciampi et al. (Ciampi et al. 2003) show that carefully designed ventilated facades, walls and roofs can reduce considerably the summer thermal loads.
The advantage of integrated photovoltaic over non-integrated systems is the reduction of construction costs of building materials. These advantages make BIPV one of the fastest growing segments of the photovoltaic industry (Park et al. 2010). For BIPV systems to achieve multifunctional roles, various factors must be taken into account, such as the photovoltaic module temperature, shading, installation angle and orientation, effect integration on building thermal comfort (Peng et al. 2011). Infield et al. (Infield et al. 2004) applied a steady state analysis model in a ventilated PV facade in order to evaluate an overall heat loss coefficient and thermal gain factor and suggested that the temperature of the PV module can be reduced by flowing air between the PV module and the double glass wall. Similar studies were carried out by (Tripanagnostopoulos et al. 2002), (Zondag et al. 2002), (Prakash 1994) and (Chow et al. 2003) by flowing air and water below the PV module to increase the electrical efficiency of the PV module. (Tiwari et al. 2006) have evaluated the performance of the photovoltaic (PV) module integrated with air duct for composite climate of India. Analytical expression for overall energy efficiency (electrical and thermal) has been derived. It is observed that there is a fair agreement between theoretical and experimental observations and concluded that an overall energy efficiency of photovoltaic thermal (PVT) system is significantly increased by utilization of thermal energy in PV module. (Agrawal & Tiwari 2010) optimized the opaque type BIPVT system for cold climatic conditions. The system fitted on the roof top of Srinagar over an effective area of 65 m2 produces annually the electrical and thermal exergy of 16209 kWh and 1531 kWh. (Vats & Tiwari 2012) evaluate a building integrated semitransparent photovoltaic thermal (BISPVT) system and found for an effective area of 5.44 m2 overall annual thermal energy gain is 2497 kWh and electrical gain is 810 kWh. (Ekoe et al. 2015) proposed a thermal modelling of a semi-transparent PV module with fins at the back surface. It is observed that the system exhibits higher thermal and electrical efficiencies than the conventional BISPVT systems, the maximum value of cell temperature can decreased from 62.68 °C to 53.75 °C and heat extracted by air in a year from the fin surface is amounts 55.4 kWh/year. The results of basic studies on irradiance and energy output of photovoltaic systems have been reported by some researchers (Rahman et al. 1988; Celik 2003; Smiley 2001) while there have been other studies on the temperature and generation performance of photovoltaic modules (Mattei et al. 2006; Carr & Pryor 2004; Chenni et al. 2007) integrated in building.
The humidity is among the indoor environmental factors that could affect thermal comfort in a building. Accurate prediction of indoor conditions, specifically indoor temperature and relative humidity, are important for the following four reasons: 1) To better assess the hygrothermal performance of building envelope components (Tsongas et al. 1996; Tariku et al. 2009) and reduce the likelihood of building envelope failure. High indoor humidity can result in excess moisture accumulation in the structures and deterioration of components due to mold/decay or corrosion. 2) To maintain the critical relative humidity range, which is specific to the building’s operation (Trechsel 2001; Rode 2003). The relation between relative humidity of air and indoor air temperature was developed by Djamila in the Malaysia climate environment for indoor temperature range of 27 °C to 35 °C (Djamila et al. 2014). Hartwig summarizes the findings concerning indoor relative humidity, and specifications, which are partly derived from standards or guidelines for the hygrothermal design of building components Hartwig et al.
When solar photovoltaic system is integrated in building, indoor air temperature and humidity (IATH) are changed due to the modification of thermal resistance of building envelope. There are not many research focused on that ways. In this paper, the effect of the BIPV on the indoor air temperatures and humidity (IATH) of a multiple storey buildings under the tropical climatic conditions of Yaoundé, Cameroon has been modelled and analysed. Two cases of BIPV made of 290 m2 area of PV have been considered, i) roof integrated and ii) façade integrated. In addition, building orientation, roof pitch and the building materials are also been explored and optimised to provide the best combination.
The building in Yaoundé, Cameroon situated at 3°852' N, 10°83' E is under consideration. The size of the building is 26.65 m × 22.6 m with an average height of 15.3 m. The building is insulated by the layer of sand and cement. The roof top cover an effective area of (375.8 × 02) m2 and an effective area on façade are respectively 348.15 m2 on the top side and behind, 396.15 m2 on the right and the left side. Photovoltaic system is integrated on the roof or at facade. Figure 1 shows the pictorial view of building and the BIPV system. The choice of roof inclination and building orientation are derived from overall building air temperature calculation.
BIPV integration on roof top (a) and on façade (b)
The heat flow through a building construction depends on the temperature difference across it, the conductivity of the materials used and the thickness of the materials. The temperature difference is an external factor. The thickness and the conductivity are properties of the material. These parameters form the thermal resistance of the construction, given as:
The total resistance of an element includes all of the resistances of the individual materials that make it up as well as both the internal and external air-film resistance. According to the material used, theirs dimensions and theirs thermal properties, each part of building envelop will have its own overall heat transfer coefficient.
Relative humidity indoors has a very important role in respect of indoor air quality, thermal comfort, occupant health, material emissions and energy consumption. A too low relative humidity indoors may cause respiratory illnesses and asthma. However, also too high relative humidity has negative effects such as mould and moisture problems, dust mites and it might also cause respiratory illnesses (Airaksinen 2013). The following formula gives the Relative Humidity (Morel & Gnansounou 2009):
Where Td the dew point temperature of vapor water is given as:
The water vapour saturation pressure to sufficient accuracy between 0 °C and 373 °C is given as (Wagner & Pruß 2002):
For the evaluation of the influence of the changes in the thermal resistance of the building envelope on the temperature of the internal air of the building, the following steps are considered:
The average temperature of the building and its different floors are determined for the building without BIPV integration.
The BIPV system is integrated on facade, and the average hourly temperature variation of the building is determined, as well as different floors of the building. This is made for different rotations of 90° to the front side corresponding to the North, East, South and West orientation. For the orientation corresponding to the maximum and minimum temperature obtained, the type of main walls of the building material is modified by those embedded in the customs of local buildings in Cameroon.
BIPV system is integrated in the roof, and the average hourly temperature variation of the building is determined, as well as the different floors of the building. This is made for different rotations of 90° to the front side corresponding to the North, East, South and West orientation. For the orientation corresponding to the maximum and minimum temperatures, calculations are also performed for an inclination of the roof from 5° to 40°, then the nature of the main roof material is changed.
The relative humidity is obtained with the help of Eq. (2, 3, 4).
The simulation is performed with the Designbuilder software that integrates the powerful calculation tool energyplus.
In order to obtain the dynamic behaviour of the system, as well as estimating the internal building air temperature for hot season of the year, we used global and diffuse solar radiation data of a representative day over Yaoundé region for the year 2012 obtained from the Energy and Environmental Technologies Laboratory of the Department of Physics at the University of Yaoundé I; we also used climatic data issued by the Atmospheric Physics Laboratory.
Table 1 summarises the properties of building materials. The overall heat transfer coefficient U calculated with energyplus convection algorithm take account the convective heat transfer coefficient of internal and external surface (2.152, 19.87) W/m2K and the radiative heat transfer coefficient of internal and external surface (5.54, 5.13) W/m2K.
Table 1
Buildings materials properties
| e | U | ||
|---|---|---|---|
| Roof | Aluminum sheet | 0.001 | 7.143 |
| Plywood | 0.004 | 3.371 | |
| Wall | Cast concrete/Concrete block/ Cast concrete | 0.015 + 0.17 + 0.015 | 0.833 |
| Cast concrete/breeze bloc/Cast concrete | 0.015 + 0.17 + 0.015 | 2.450 | |
| Cast concrete/brickwork/cast concrete | 0.015 + 0.17 + 0.015 | 2.507 | |
| Cast concrete/Ashlar stone/Cast concrete | 0.015 + 0.17 + 0.015 | 3.643 | |
| Woods | 0.2 | 0.626 | |
| Rammed earth, (adobe) | 0.2 | 3.065 | |
| Openings | Wooden door | 0.03 | 2.393 |
| Glass Window | 0.01 | 3.571 | |
The values of the design parameters of BIPV facade integration are given in Table 2 and Fig. 2 shows the pictorial view of building integrated photovoltaic system.
Table 2
Design parameters of BIPV facade integration
| e | U | ||
|---|---|---|---|
| On wall | PV module | 0.01 | 3.571 |
| Concrete block | 0.015 | 4.774 | |
| Glass/insulation | 0.004 | 3.226 | |
Building integrated photovoltaic system (a) at facade and (b) at roof
The variations of global and diffuse solar radiation data of Yaoundé for the year 2012 obtained from the Environmental Energy Technologies Laboratory (EETL) at the University of Yaoundé I and the average temperature are shown in Fig. 3.
Average solar radiation and ambient temperature
The main floor building (RDC), floor 1 to 4, roof and average building hourly temperature are shown in Fig. 4.
It is observed that the air temperature inside the building increases with the floor level. The indoor air temperature of the main floor building and floor 1 to 3 is substantially equal to the outside air temperature. However, from the fourth floor, indoor air temperature of the building is higher than the outside temperature, from 9:00 am to 11:00 pm with a maximum temperature difference of 3 °C à 06:00 pm.
Building indoor air temperatures without BIPV installed
It is observed from the Fig. 5 that the relative humidity of indoor air of buildings varied between 70.89 % and 85.45 %. The indoor relative humidity decrease when the temperature of air inside the building increases. A temperature difference of 2 °C corresponds to a relative humidity difference of 1 %. The variation of hourly air temperature of the main floor building, floor 1 to 4 and roof for facade bipv integration are presented in Fig. 6.
Indoor air relative humidity (a) and its comparison with temperature (b)
Hourly temperature variation of indoor air of the main floor building (a), floor 1 to 4 (b) – (e) and roof (f) for BIPV facade integration with building orientation
The integration of PV modules in the structure of the building envelope modified the indoor air temperature of the building, with a maximum temperature difference of 3.15 °C observed at 12:00 am. It is also observed that this integration has a negligible effect on the indoor air temperature of the roof.
The level of natural ventilation in a building depends on the direction and the speed of the wind, the orientation of the construction and its opening. Natural ventilation also influence the temperature of the indoor air of the building by renewing the indoor air with an almost constant and regular flow rate. This attenuation is maximal when the openings create a horizontal flow of air.
The maximum temperatures differences induced by this type of integration are observed on the floor 3 when the BIPV area is oriented East side, while the minimum temperatures are obtained with BIPV is North orientated. Figure 7 shows the hourly temperature of floor 3 for different types of the principal wall materials.
Hourly temperature variation of indoor air of floor 3 for different type of the principal wall material for bipv East orientation (a) and bipv Nord orientation (b)
By a combination of the orientation with the type of the principal wall material, temperature difference can be up to 4 °C obtained with the wood as the main material of the wall.
The variation of hourly temperatures of the main floor building, floor 1 to 4 and roof for BIPV roof integration with building orientation are presented in Fig. 8
Hourly temperature variation of indoor air of the main floor of building (a), floor 1 to 4 (b) – (e) and roof (f) for BIPV roof integration with building orientation
The integration of photovoltaic modules on the roof of the building modified the indoor air temperature of the roof and the floor on which it is based, namely the 4th floor, with a maximum temperature difference of 2.7 °C at 12:00 am. The temperature is maximum when the PV system is oriented south, while the west orientation provides minimum temperatures for roof integration.
Figure 9 shows the hourly variation of the internal temperature on the floor 4 with roof inclination of 5° to 40° for the southern and western orientation.
hourly temperature variation of indoor air of building floor 4 with roof inclination for south (a) and west (b) orientation
A comparison between the two types of BIPV integration is made through Fig. 10 which shows the variation of the average hourly indoor air temperature of the building.
Average hourly indoor air temperature of the building for BIPV facade (a) and roof (b) integration
It is observed that the BIPV roof integration is more advantageous than the BIPV façade integration:
The influence of a building integrated photovoltaic system (BIPV) on room air temperature in tropical region has been analysed. The comparison of room air temperature derived from roof integration and façade integration is made for the same area in residential buildings, orientation of buildings, roof inclination and building materials are also explored. It has been observed that:
The principal parameter of BIPV is building orientation, the increase of building indoor air temperature due to BIPV façade integration can be considerably reduced by the openings if they create a horizontal flow of air.
e Thickness (m)
RH Relative Humidity (%)
k Thermal conductivity (W/m.K)
P Pressure, (hPa)
R Thermal resistivity (K/W)
S Area (m2)
T Temperature (K)
U Overall heat transfer coefficient (W/m2K)
Greek symbols
φ Inclination of roof (rad)
Subscripts
BIPV Building Integrated Photovoltaic System
C Critical, constant
d Direct, Dew point
IATH Indoor Air Temperature and Humidity
HVAC Heating, ventilating and air Conditioning
g Global
RDC main floor of building, ground floor
v vapor Water
ws Water vapour saturation
Agrawal, B and Tiwari, GN Optimizing the energy and exergy of building integrated photovoltaic thermal (BIPVT) systems under cold climatic conditionsAppl Energy201087417426https://doi.org/10.1016/j.apenergy.2009.06.011
Airaksinen, M The Influence of Moisture Capacity of Building Materials on the Indoor Air QualityInt J Environ Prot201337110
Al-Homoud, MS Thermal design of office buildingsInt J Energy Res19972110941957https://doi.org/10.1002/(SICI)1099-114X(199708)21:10<941::AID-ER302>3.0.CO;2-Y
Bodart, M and Herde, A Global energy savings in offices buildings by the use of day lightingEnergy Build2002345421429https://doi.org/10.1016/S0378-7788(01)00117-7
Carr, AJ and Pryor, TL A comparison of the performance of different PV module types in temperate climatesSol Energy200476285294https://doi.org/10.1016/j.solener.2003.07.026
Celik, AN Long-term energy output estimation for photovoltaic energy systems using synthetic solar irradiation dataEnergy2003285479493https://doi.org/10.1016/S0360-5442(02)00140-82015403
Chenni, R, Makhlouf, M, Kerbache, T and Bouzid, A A detailed modeling method for photovoltaic cellsEnergy200732917241730https://doi.org/10.1016/j.energy.2006.12.006
Chow, TT, Hand, JW and Strachan, PA Building-integrated photovoltaic and thermal applications in a subtropical hotel buildingAppl Therm Eng20032320352049https://doi.org/10.1016/S1359-4311(03)00183-2
Ciampi, M, Leccese, F and Tuoni, G Ventilated facades energy performance in summer cooling of buildingsSol Energy200375491502https://doi.org/10.1016/j.solener.2003.09.010
Djamila, H, Chu, C-M and Kumaresan, S Effect of Humidity on Thermal Comfort in the Humid TropicsJ Build Construction Plann Res20142109117https://doi.org/10.4236/jbcpr.2014.22010
Ekoe A Akata AM, Njomo D, Agrawal B (2015) Thermal energy optimization of building integrated semi-transparent photovoltaic thermal systems. Int. J Renew Energy Dev 4(2):113–123. dx.doi.org/10.14710/ijred.4.2.113-123
Hartwig M. Künzel. Indoor Relative Humidity in Residential Buildings – A Necessary Boundary Condition to Assess the Moisture Performance of Building Envelope Systems. Download: http://www.hoki.ibp.fraunhofer.de/ibp/publikationen/fachzeitschriften/wksb%20Raumluftfeuchte1_E.pdf
Hestnes, AG Building Integration of Solar Energy SystemsSol Energy199967181187https://doi.org/10.1016/S0038-092X(00)00065-7
Infield, D, Mei, L and Eicker, U Thermal performance estimation for ventilated PV facadesSol Energy2004769398https://doi.org/10.1016/j.solener.2003.08.010
Iqbal, I and Al-Homoud, MS Parametric analysis of alternative energy conserva-tion measures in an office building in hot and humid climateBuild Environ200742521662177https://doi.org/10.1016/j.buildenv.2006.04.011
Jelle, BP, Breivik, C and Røkenes, HD Building integrated Photovoltaicproducts: astate-of-the-art review and future research opportunitiesSol Energy Mater Sol Cells20121006996https://doi.org/10.1016/j.solmat.2011.12.016
Kimura, K-i Photovoltaic systems and architectureSol Energy Mater Sol Cells199435409419https://doi.org/10.1016/0927-0248(94)90168-6
Lee, SE and Selkowitz, ES The New York Times headquarters day lighting mockup: monitored performance of the day lighting control systemEnergy Build2006387914929https://doi.org/10.1016/j.enbuild.2006.03.019
Mattei, M, Notton, G, Cristofari, C, Muselli, M and Poggi, P Calculation of the polycrystalline PV module temperature using a simple method of energy balanceRenew Energy200631553567https://doi.org/10.1016/j.renene.2005.03.010
Mehlika, N, Inanici, F and Nur, D Thermal performance optimisation of buildingaspect ratio and south window size in five cities having different climaticcharacteristics of TurkeyBuild Environ20003514152https://doi.org/10.1016/S0360-1323(99)00002-5
Mohd Shahidan Shaari, Hafizah Abdul Rahim, Intan Maizura Abd Rashid (2013) Relationship among population, energy consumption and economic growth in malaysia. Int J Soc Sci. Vol. 13, No. 1
Park, KE, Kang, GH, Kim, HI, Yu, GJ and Kim, JT Analysis of thermal and electrical performance of semi-transparent photovoltaic (PV) moduleEnergy20103526812687https://doi.org/10.1016/j.energy.2009.07.019
Peng, C, Huang, Y and Wu, Z Building - integrated photovoltaics (BIPV) in architectural design in chinaEnergy Build20114335923598https://doi.org/10.1016/j.enbuild.2011.09.032
Prakash, J Transient analysis of a photovoltaic-thermal solar collector for co-generation of electricity and hot air/waterEnergy Convers Manag199435967972https://doi.org/10.1016/0196-8904(94)90027-2
Rahman, S, Khallat, MA and Salameh, ZM Characterization of insolation data for use in photovoltaic system analysis modelsEnergy19881316372https://doi.org/10.1016/0360-5442(88)90079-5
Rode, C Whole Building Hygrothermal Simulation Model. ASHRAE Transactions2003
Smiley, EW Low irradiance performance modelling for building integrated photovoltaics17th European PV Solar Energy Conference, Munich, Germany2001
Stegou-Sagia, A, Antonopoulos, C, Angelopoulou, C and Kotsiovelos, G The impact of glazing on energy consumption and comfortEnergy Convers Manag2007481128442852https://doi.org/10.1016/j.enconman.2007.07.005
Taleb, HM and Pitts, AC The potential to exploit use of building-integrated photovoltaics in countries of the Gulf Cooperation CouncilRenew Energy20093410921099https://doi.org/10.1016/j.renene.2008.07.002
Tariku, F, Kumaran, MK and Fazio, P The need for an accurate indoor humidity model for building envelope performance analysis2009
Tiwari, A, Sodha, MS, Chandra, A and Joshi, JC Performance evaluation of photovoltaic thermal solar air collector for composite climate of IndiaSol Energy Mater Sol Cell200690175189https://doi.org/10.1016/j.solmat.2005.03.002
Trechsel, HR Moisture Primer, Moisture Analysis and Condensation Control in Building Envelopes, ASTM Manual series 50: Chapter 12001
Tripanagnostopoulos, Y, Nousia, T, Souliotis, M and Yianoulis, P Hybrid photovoltaic/thermal solar systemsSol Energy200272217234https://doi.org/10.1016/S0038-092X(01)00096-2
Tsongas, G. Burch, D. Roos, C. Cunningham, M. (1996). A Parametric Study of Wall Moisture Contents Using a Revised Variable Indoor Relative Humidity Version of the 52 MOIST Transient Heat and Moisture Transfer Model. Proceedings of the Thermal Performance of Exterior Envelopes VI Conference, Dec. 4–8, Clearwater Beach, FL
Vats, K and Tiwari, GN Energy and exergy analysis of a building integrated semitransparent photovoltaic thermal (BISPVT) systemAppl Energy201296409416https://doi.org/10.1016/j.apenergy.2012.02.079
Wagner, W and Pruß, A The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific UseJ Phys Chem Ref Data2002312387535
Wong, NH, Wang, L, Chandra, AN, Pandey, AR and Wei, X Effects of double glazed facade on energy consumption, thermal comfort and condensation for a typical office building in SingaporeEnergy Build2005376563572https://doi.org/10.1016/j.enbuild.2004.08.004
Zain-ahmed, A, Sopian, K, Othman, MYH, Sayigh, AAM and Surendran, PN Day lighting as a passive solar design strategy in tropical buildings: a case study of MalaysiaEnergy Convers Manag2002431317251736https://doi.org/10.1016/S0196-8904(01)00007-3
Zhai, XQ, Wang, RZ, Dai, YJ, Wu, JY and Ma, Q Experience on integration of solar thermal technologies with green buildingsRenew Energy20083319041910https://doi.org/10.1016/j.renene.2007.09.027
Zondag, HA, Vries, DW, Helden, WGJ, Zolingen, RJC and Steenhoven, AA The thermal and electrical yield of a PV-thermal collectorSol Energy200272113128https://doi.org/10.1016/S0038-092X(01)00094-9
C1 = −7.85951783
C2 = 1.84408259
C3 = −11.7866497
C4 = 22.6807411
C5 = −15.9618719
C6 = 1.80122502
Pc = 220640 hPa
Tc = 647.096 K