Analysis of simplified calculation procedures of solar gain in buildings

G. Oliveti, N. Arcuri, M. De Simone, R. Bruno,

Department of Mechanical Engineering — University of Calabria
87036 — P. Bucci 44/ C — Rende (CS) — ITALY
M. De Simone, marilena. desimone@unical. it

Abstract

A critical analysis of the calculation procedure of solar gain in buildings with reference to an attached sunspace bordering air conditioned environments was developed. The EN ISO 13790:2008 Standard provides a simplified calculation method for the estimation of solar gain based upon the monthly energy balance in a stationary regime, to be applied both during winter heating and summer cooling. Moreover, the Standard hypothesises possible simplifications to be evaluated in relation to climatic conditions and the type of greenhouse-environment system. The direct, indirect and total solar contribution evaluated by means of the Standard are compared with those obtained with a dynamic calculation code, with the aim of highlighting the limits of the schematization adopted by the Standard. Furthermore, the gain in an airconditioned environment obtained by means of a windowed wall is compared with that produced by the use of a sunspace opposite the same wall.

Keywords: Energy balance, solar heat gain, Standard, sunspace.

1. Introduction

The use of solar radiation in buildings is of particular interest in the case of the use of glazed systems such as glazed balconies and sunspaces. Sunny spaces adjacent to air conditioned volumes are passive solar systems, usable in order to reduce the thermal energy requirement of adjacent spaces in winter and to control solar gain in the summer months.

For solar contribution we are refering to the energy that, in a determined time interval, reaches the airconditioned environment by means of the separating walls of the lodge or the greenhouse. Such a gain is produced by solar radiation which directly penetrates the environment through the glass surfaces, and the energy transmitted by conduction through the dividing walls consequent to the absorption of solar radiation. The energy absorbed by the greenhouse shell is in part given to the adjacent rooms, in part transferred to the outside, in part removed by the ventilation flow and in part remains accumulated within the opaque walls [1].

The EN ISO 13790:2008 Standard [2] in Annex E provides a calculation method of solar gain obtainable with specific elements including unconditioned sunspace attached to buildings; it confirms the UNI EN 832:2001 calculation method [3] based upon the monthly energy balances in a stationary regime and also extends its validity to the summer. Furthermore, the EN ISO 13790 suggests conservative simplifications in calculating the monthly energy requirements of the adjacent environment whose validity requires checking in different national realities.

The Standard method distinguishes between direct solar contribution which reaches the airconditioned environment through the glazed and opaque surfaces which separate the sunspace from the environment, and indirect solar contribution deriving from heating of the air in the sunspace. For the evaluation of direct contribution, consider the energy entering through the glass separation surface of the sunspace-environment completely absorbed by the environment (black cavity hypothesis), and the energy absorbed by the opaque separation components, which is conducted in the walls, evaluated by a simplified model that does not consider the effects of thermal capacity.

The estimation of indirect solar contribution is obtained considering the solar energy absorbed by only the opaque components of the sunspace, after the conductive transfers in the conditioned environments, which one is supposed to be transferred to the greenhouse air.

In this article, by means of the DEROB-LTH (Dynamic Energy Response of Buildings) dynamic simulation code [4], a critical analysis was developed of the procedures used by the EN ISO 13790 Standard for the calculation of direct and indirect solar contribution, and furthermore, with reference to the Italian climate, verifies the possibility of adopting some simplifications proposed by the same Standard relating to the heating and the cooling. The system considered is an attached sunspace separated from an adjacent airconditioned environment by a mixed opaque-glazed wall, and bordering by means of the flooring with a second airconditioned space. The DEROB-LTH code resolves the transmission of solar radiation through the glazed surfaces taking into consideration the directional aspects, and the absorption in the greenhouse and the adjacent environment considering the directly irradiated portions of the surfaces and the redistribution of solar radiation consequent on reflection phenomena. Moreover, the code resolves the thermal field in the glazed and opaque walls of the sunspace taking into consideration the effective surrounding conditions that act upon both the internal and external surfaces [5].

The comparison of monthly energy was carried out in the same application conditions of the Standard, that is to say considering only solar power as an agent on the sunspace. This was obtained in the simulations by eliminating the other powers, or rather by imposing the same temperature constant as the aircondtioned environment for the external air and the sky. The simulations were conducted on an hourly basis using both direct and diffuse radiation values relative to the average monthly day [6].