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

The energy performance of a radiant ceiling system used for the heating and cooling of an open space environment with a surface area of 800 m2 and an air-conditioned volume equal to 3200 m3 is presented. In winter, the radiant ceiling is supplied by an integrated field of solar collectors with an auxiliary source. During summer, the solar collectors are used to supply the generator of a closed cycle simple effect absorption machine which produces, along with a traditional auxiliary heat pump, the refrigerated flow rate for radiant ceilings. The control system regulates the temperature and the supplied flow rate of the radiant ceilings with different logics during winter and summer periods, intervening directly on the solar circuit. The entire building-plant system was simulated with the TRNSYS code using climatic data for the location of Cosenza (Lat. 39.18N) in order to identify the energy performance varying the collectors surface and storage volume.

Keywords: Thermal solar collectors plant, Absorption Chiller, Control strategy

1. Introduction

The energy performance of a plant which permits the heating and cooling of a building which uses a solar radiation capturing field, a storage tank, a cooling tower and a simple effect absorption machine using a water-lithium bromide mix was evaluated. From the storage tank a flow rate of hot water feeds the radiant ceiling in winter, and the generator of the absorption machine in summer. The existing absorption machine has a nominal power of 35 kW, and requires a cold water flow rate to the condenser and absorber obtained by means of a cooling tower. A three way valve system permits the regulation of the temperature of the flow rate used during winter heating, and of the flow rate used by the generator of the absorption chiller. The plant is equipped with an auxiliary heater which is only active in winter, linked parallelly to a storage tank, which supplies the flow rate at the required temperature when the temperature of the water in the tank is insufficient [1].

For the distribution of thermal energy, a radiant ceiling plant was chosen, characterised by limited thermic inertia and which requires moderate surface temperatures [2].

The building-plant system was studied in winter and in summer with the TRNSYS dynamic simulation code [3], which permitted the modelling of various plant components and the implementation of a specially created control procedure. The building considered is situated in Cosenza, a location in southern Italy with Mediterranean type weather, characterised by not too severe winters and hot, relatively humid summers [4]. In order to simulate the effective climatic variability, a procedure of hourly values of solar irradiation and external air temperature were used, beginning from respective average monthly data [5].

2. Description of the building-plant system

The plant considered represented in Figure 1 is formed by a primary solar circuit and by a secondary circuit which supplies the radiant ceiling. The primary circuit is formed by a field of

heat pipe solar collectors, by an extraction pump (a) from the storage tank, and by a thermal relieve valve which guarantees safety conditions in the plant. The solar collectors are inclined at 20°, in that with such an incline the solar irradiation recorded during summer is at the maximum and represents the minimum limit in order to guarantee the correct functioning of the heat pipe solar collectors.

The secondary circuit is formed by elements involved both in winter and summer functioning, by elements which function only in heating or cooling applications and by three three way valve systems. During winter, the first three way valve system (1) permits, by means of the activation of a pump (b), the mixing of the flow rate extracted from the tank with the flow rate exiting the radiant ceiling plant, for the regulation of the inlet temperature in the case in which the water temperature within the storage tank is higher than that required by the control system [6]. The flow rate exiting the absorption chiller coming from the circuit with the second three way valve system (2) is only active in summer. In such a period the two valves (1), by means of the activation of a pump (d), mix the flow rate extracted from the storage tank with that exiting the absorption chiller generator in the case in which the tank temperature is higher than that required by the control system of the absorption machine, in such a way as to provide the required refrigerating power [7]. The third three way valve system (3) completes a parallel circuit, which only functions in winter, which activates the auxiliary system in order to provide the entire supplied flow rate at the desired temperature in the case in which the temperature within the tank is lower than that required by the control system.

The hot or cold water flow rate produced in the plant supplies a radiant ceiling by means of a group of plastic parallel pipes, with thermal conductivity equal to 0.35 Wm"1K"1, embedded in the covering plaster of the upper floor having a thickness of 2 cm. The internal diameter of the pipes is 8 mm and their pitch is10 cm; in each pipe a flow rate of 36.9 kg/h circulates which guarantees a velocity of about 0.2 m/s. In such conditions the mass flow rate regime is laminar. The radiant ceiling system is situated in an open-space environment with an air-conditioned volume equal to 3200 m3, parallelepiped in shape with four vertical dispersant walls, and by an upper delimitation flooring that is also dispersant. In Table 1 the structural and thermo-physic properties of the walls are listed. The vertical windowed surfaces cover an area equal to 50% of the total surface area of the opaque wall. They are made of a metallic frame equipped with a thermal cut and double glazed system, with a transmittance of 2.8 Wm’2K_1 and a solar gain gx of 0.75. In order to limit the incidence of loads during the summer, the South, East and West facing windows are equipped with external shading devices which minimize the incident solar radiation by 50%. The activation of the shading devices is illustrated in Table 2. During the day within the building the endogenous loads produced by the presence of people, a number varying between 48 and 96 with a pro-capita generation of 65 W of sensible heat and 55 W of latent heat were taken into consideration. Furthermore, 96 personal computers were taken into consideration which provide a sensible unitary power of 140 W and an artificial illumination system, lit from 08.30 to 18.30 which delivers a sensible heat flux of 5 Wm-2.

According to Italian standards [8], for this type of building a crowding index of 0.12 people per surface unit and a pro-capita external air change of 11 ls-1 is foreseen, which gives rise to a total external air renewal load equal to about 1.2 Volh-1. The task of removing latent loads and regulating the specific humidity within the environment was assigned to the external air flow rate.