The Role of the Internal Heat Gains for Artificial Lighting on the Energy Performance of Buildings Guglielmina Mutani 1* , Edoardo De Nicolò 1 , Laura Blaso 2 , Simonetta Fumagalli 2 , Antonella Tundo 3 1 R3C, Department of Energy, Politecnico di Torino, c.so Duca degli Abruzzi, 24, Torino 10129, Italy 2 ENEA Smart Cities and Communities Laboratory, Smart Energy Division, Department of Energy Technologies and Renewable Sources - via Enrico Fermi, at JRC, 2749, Ispra (VA) 21027, Italy 3 ENEA Smart Cities and Communities Laboratory, Smart Energy Division, Department of Energy Technologies and Renewable Sources - via Giulio Petroni, 15 f, Bari 70124, Italy Corresponding Author Email: gugliemina.mutani@polito.it https://doi.org/10.18280/ijht.390501 ABSTRACT Received: 17 July 2021 Accepted: 22 October 2021 This paper aims to propose a procedure for calculating the energy performance indexes of buildings considering the seasonality of internal gains due to artificial lighting with a monthly quasi-steady-state energy balance. The proposed methodology evaluates the heat gains due to the integrated natural-artificial lighting system with the Lighting Energy Numerical Indicator (LENI). For the evaluation of buildings’ global energy performance and for some energy services, this contribution cannot be considered constant annually as depend strongly by climate conditions. The effect of daylighting, type of light sources- luminaires, building orientation and shading devices could influence lighting contribution of the internal heat gains. Then, the proposed methodology evaluates the internal heat gains with monthly energy balances. This methodology was applied to the case study of the "Brancaccio" retirement home in Matera (IT) for which the values of the energy performance indexes were compared with the standard normative approach using constant internal heat gains. The results of this work underline the importance of performing a detailed analysis that considers the availability of natural light in the different months of the year, the efficiency of the different lighting systems and their power installed per unit of area as a function of the lighting comfort requirements in the different types of environments. Keywords: LENI, artificial lighting, energy performance indicator, energy certification, heat gains, light control systems, nZEB, smart buildings 1. INTRODUCTION The energy consumption of buildings has a strong environmental impact and has been widely debated and constantly a developing topic. The European Union has issued a series of specific directives that require the verification of the energy performance of all buildings (existing and new construction) in terms of global consumption, with limit values not to be exceeded. Electrical energy covered an important quota of final energy consumption and lighting can be a significant part of this electrical consumption, especially in residential buildings [1]. In Italy, the Ministerial Decree of 26/06/2015, which entered into force on October 1 st 2015, introduced important innovations regarding energy efficiency and low-carbon technologies. Among these, the energy class of buildings must be assessed in terms of non-renewable primary energy through the global non-renewable energy performance index EPgl,nren as the sum of the non-renewable energy indexes for each energy service [2, 3]: gl,nren H,nren C,nren W,nren V,nren T,nren L,nren EP =EP +EP +EP +EP +EP +EP (1) This energy performance index EP is expressed in kWh/m 2 /year so as to consider only the level of energy efficiency of the building and not its size. It takes into account the non-renewable primary energy requirement for all energy services in a building: space heating and cooling (EPH,nren and EPC,nren), hot water production (EPW,nren), ventilation (EPV,nren), and, for non-residential sectors, transport of people or things (EPT,nren) and artificial lighting (EPL,nren). The annual energy performance indicator EPgl,nren is also used to classify the buildings with an energy performance certification [4, 5] and the EPgl,nren is therefore calculated through methodologies codified by technical standards and national legislation, in transposition of European directives. The “family” of UNI/TS 11300 Technical Standards was created with the aim of defining a unique calculation methodology for determining the energy performance of buildings with a monthly quasi-steady-state calculation method. In these six Standards, the internal heat gains, due to people, electrical equipment and lighting systems, are evaluated through a constant annual average value depending on the type of user. In this work, this approach is investigated in particular with regard to the role played by heat gains due to the lighting system [6]. The use of a standardized average value [7] does not consider crucial factors such as the differences in: the availability of natural light in the various months of the year and for different climates [8], the effect of shading devices, the efficiency of the lighting systems and the electrical power installed per unit of surface area as a function of the lighting comfort requirements in the different International Journal of Heat and Technology Vol. 39, No. 5, October, 2021, pp. 1395-1404 Journal homepage: http://iieta.org/journals/ijht 1395