What is the effect of optimum independent parameters on solar heating systems? Erkan Kaçan a,⇑ , Koray Ulgen b , Erdal Kaçan c a Department of Electric and Energy, Pamukkale University, Faculty St. No: 30 Camlık Campus, Denizli, Turkey b Solar Energy Institute, Ege University, 35100 Bornova, Izmir, Turkey c Department of Chemistry and Chemical Technologies, Pamukkale University, Faculty St. No: 30 Camlık Campus, Denizli, Turkey article info Article history: Received 15 May 2015 Accepted 18 July 2015 Keywords: Central Composite Design (CCD) Environmental efficiency Exergy analysis Response surface methodology (RSM) Solar Combisystem Solar heating abstract Researchers are rather closely involved in Solar Combisystems recently, but there is lack of study that presents the optimum design parameters. Therefore, in this study the influence of the four major variables, namely; outside, inside temperature, solar radiation on horizontal surface and instantaneous efficiency of solar collector on the energetic, exergetic and environmental efficiencies of Solar Combisystems are investigated and system optimization is done by a combination of response surface methodology. Measured parameters and energetic–exergetic and environmental performance curves are found and statistical model is created parallel with the actual data. It is found that statistical model is significant and all ‘‘lack-of-fit’’ values are non-significant. Thus, it is proved that statistical model strongly represents the design model. Outside temperature, solar radiation on horizontal surface and instantaneous efficiency of solar collector are the decisive parameters for all responses but instantaneous efficiency of solar collector is not for environmental efficiency. Maximum exergetic efficiency exceeded 11%, maximum environmental efficiency reached up to 95% at optimized process. The optimum value of the outside temperature and solar radiation are found as 22 °C and 773 W/m 2 for all responses, on the other hand optimum collector efficiency is found around 60% for energetic and exergetic efficiency values. Inside temperature is not a decisive parameter for all responses. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Solar heating systems are all related with the purpose of an action to reduce the energy consumption in buildings and even in industry [1]. Energy consumption in industry has an important role with respect to sustainability and competitivity in the market [2]. Each system design aims to increase the fractional solar con- sumption value (FSC) and to reduce the consumed auxiliary energy which are usually selected as fossil fueled sources. The energy con- sumption values in buildings should be underlined in order to pre- sent the importance of the role any system which is designed to decrease the energy consumption in buildings and/or provide heat- ing demand via renewable energy sources. Distributed energy consumption values can be seen in Fig. 1 and final energy consumption in residential areas occupies an impor- tant place [3]. It is reported that approx. 40% of final energy consumption occurs in buildings in EU members. This ratio means that 36% of greenhouse gas emissions are released from buildings in 2013 [4]. The percentage of final energy consumption in buildings is above the critical threshold value for future projection of the EU. That is why European Commission created future projections for 2020 energy efficiency target of EU with respect to energy effi- ciency investments as a financial action. Energy consumption value in buildings were reported as 20– 40% of total energy consumption for developed countries in 2004 [5]. This increment can be explained by population growth, rising urbanization and energy intensity in central places [6]. Nearly same values were announced from U.S. Energy Information Administration (U.S. EIA), 41% of total U.S. energy con- sumption was consumed in residential and commercial buildings in 2014 [7]. For Turkey, one third of total energy consumption occurs in buildings and 85% of this energy consumes for heating purpose in 2010 [8]. As we consider Turkey as a developing country, energy consumption in buildings has a vital interest for both developing and developed countries [9]. http://dx.doi.org/10.1016/j.enconman.2015.07.052 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +90 258 212 37 88x1207; fax: +90 258 211 80 65. E-mail addresses: erkankacan@pau.edu.tr (E. Kaçan), koray.ulgen@ege.edu.tr (K. Ulgen), ekacan@pau.edu.tr (E. Kaçan). Energy Conversion and Management 105 (2015) 103–117 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman