The design and use of climate control systems in greenhouses requires the availability of comprehensive and realistic models of all the processes involved in the mass and energy balances established between outside and inside of the greenhouse. Among them the radiative exchange in the thermal infrared range wavelength between soil, plants, cover and the night sky is usually formulated through the assumption of a constant temperature deficit of the sky relative to the environment temperature, or through preset equivalent emissivity atmospheric models whose inputs are vapor content indices. The first approximation is overly simple and the second are models not adapted to local conditions. Both put uncertainty in assessments of energy load involved in this variable. In this work, a local formulation of the equivalent sky temperature is proposed that is immediately adaptable to other models based on first principles oriented for greenhouse climate control. For this purpose the need for correction of previous approaches based on relevant local effects is justified, especially vapor scale height, the radiative terms are explicitly formulated as well as the structure of inputs and outputs, and finally value the effects of energy load models for climate control in greenhouses. Different traditional models of atmospheric thermal irradiance are compared with the proposed local model and the influence into the energy loads of a greenhouse evaluated, noting significant differences between the different models analyzed.

In this work an analysis of the potential of applying solar concentrators such as parabolic-trough solar collectors (PTC) as a thermal energy source to specific demands associated with agro-industrial processes in the province of Almeria is presented. This technology allows for an intensive utilization of solar radiation at operating temperatures superior to the ones that are possible with conventional technologies (flat plate collector, CPCs and vacuum tube) as well as covering high and medium range thermal loads with a lower solar collection area, a very favorable argument in environments characterized by a high density land use.The present work is based on a previous study of the direct solar radiation availability in the region, and in a market analysis of small modular parabolic-trough collectors that can be integrated, in terms of size and structural loads, in buildings or irregularly shaped areas. The thermal demand case selected as reference was the refrigeration of post-harvested products in fruit and vegetables packhouse and logistic centers, which in this case will be supplied by an absorption chiller assisted by solar thermal energy. A thermal load analysis was performed for a standard cooling chamber design with characteristic values of storage temperature, rotation rate and products to be cooled properties. By performing parametric dynamic simulations the area of the solar field and the storage system volume were dimensioned, obtaining various indexes that are representative of the performance of the plant.

In the present work the use of a parabolic trough solar plant to generate process heat steam for a food processing application is studied. The food processing industry, devoted to vegetables preservation by thermal treatment and canning, is located in the Southern Spain region and demands saturated steam at 7 bar with an total annual consumption of 148MWh. The base solar plant configuration analyzed consists on a parabolic trough solar field, thermally stratified energy storage, and a steam generator (unfired boiler). The influence of the main operational variables of the solar plant are studied, namely the solar field outlet temperature and the steam generator return temperature to assess its influence in the main energy based design indicators of the plant. Furthermore the possibility of including a pre-heating heat exchanger before the steam generator is also evaluated. The obtained results of this study show the suitability of changing the present energy input scheme of this industry.

This paper presents the results of the evaluation of the electricity production of a photovoltaic system integrated into the cover of a pilot greenhouse located at the experimental facilities of the Foundation ANECOOP UAL of the University of Almeria (36 degrees 51'56''' N, 2 degrees 17'2 '' N). This experimental system consists of 24 thin film opaque and flexible photovoltaic modules arranged in the roof of a greenhouse type "raspa y amagado" of 1024 m(2) with a cover of thermal polyethylene coated thickness of 200 microns and culture Daniela tomato (Solanum lycopersycum L.) with planting density of 0.75 plants/m(2). As a starting point of the work, it has been carried out an analysis of the potential of electricity production for the selected greenhouse based on incidence of global radiation at the site of the pilot installation through dynamic simulation with PVsyst. We have also analyzed other variables for the design such as the tilt and orientation of the modules and their specifications aimed to ease the integration with the selected structure, the most widespread in the province of Almeria. The experimental evaluation of the pilot system has been performed from data collected along two crop campaigns by a specific monitoring system. The conclusions of this paper can set a value of annual electricity production in terms of total area under cultivation for the studied system in the order of 8 kWh/m(2).

The increasing energy demand for air-conditioning in most industrialized countries, as well as refrigeration requirements in the food processing field and the conservation of pharmaceutical products, is leading to a growing interest in solar cooling systems. So far, the more commonly systems used are single-effect water/lithium bromide absorption chillers powered by flat-plate or evacuated tube collectors operating with COP of about 0.5–0.8 and driving temperatures of 75–95 °C. In general terms, performance of thermally driven cooling systems increases to about 1.1–1.4 using double-effect cycles fed by higher temperature sources (140–180°C). If solar energy is to be used, concentrating technologies must be considered. Although some experiences on the integration of parabolic trough collectors (PTC) and Fresnel lenses in cooling installations can be found in the literature, the quantity is far to be comparable to that of low temperature collectors. Some manufacturers have undertaken the development of modular, small, lightweight and low cost parabolic collectors, compatible for installation on the roofs of the buildings aiming to overcome some of the current technology drawbacks as costs and modularity. After a comprehensive literature review, this work summarises the existing experiences and realizations on applications of PTC in solar cooling systems as well as present a survey of the new collectors with potential application in feeding double effect absorption chillers. In addition to this, it is evaluated its use as an occasional alternative to other solar thermal collectors in air conditioning applications by dynamical simulation. Results for the case studies developed in this work show that PTC present similar levelized costs of energy for cooling than flat plate collector (FPC) and lower than evacuated tube collectors (ETC) and compound parabolic collectors (CPC).

This study deals with the characterization and analysis of the components of solar radiation under a highly diffusing cover material. measurements of outside and inside global (G(o) and G(i), respectively) and diffuse (D-o and D-i, respectively) radiation were carried out over the course of several years (2000-2004) under a plastic greenhouse. it was shown that the cover diffusive properties can be characterized in situ through sound physically-based parameters. We suggested the use of two specific greenhouse transmittances (direct-to-direct, tau(b-b), and direct-to-diffuse, tau(b-d), transmittances), or alternatively, the use of greenhouse diffuse ratio (rho=D-i/D-o) and its derivate, the enrichment coefficient (C). The transmittances were primarily dependent on the diffusive properties of the cover, with a seasonal modulation due to the influence of the beam incidence angle. We found that tau(b-d) was significantly less sensitive to the incidence angle than tau(b-b). Our results also evidenced that extrinsic factors, such as condensation events and dust deposition, might have significant non-permanent effects on both tau(b-b) and tau(b-d) during some periods of the year. Finally, we proposed a straightforward means to obtain a quantitative estimate of the amount of inside diffuse radiation through the relationship (power function) between p and the outside diffuse-to-global fraction, f(o). Importantly, the value of the exponent of the power function could be used to characterize and quantify the diffusive power of greenhouse cover materials. overall, the results stressed the significant impact of the cover diffusive properties on inside solar radiation partitioning, as well as the need to account for these changes in models aimed to estimate the direct and diffuse components, canopy radiation interception and yield in greenhouse agrosystems. (C) 2009 IAgrE. Published by Elsevier Ltd. All rights reserved.

The use of anti-insect screens placed on greenhouse vents has become generalized in warm climate areas, where there is a big pressure from insect pests which transmit virus diseases. Increasing the efficiency of the screens to reduce the entrance of pests into the greenhouse generally involves a reduction of the ventilation rate as the screen porosity is lower. This can, in part, be compensated by using screens with lower diameter of the threads which allows a higher porosity for the same efficiency to exclude insects. In the present work the characteristics of the most commonly used anti-insect screens in Almeria greenhouses, including their porosity, hole size and thread diameter were measured. Equations were developed which determined the geometric parameters of the screens accounting for their three dimensional nature. These equations enabled the effectiveness of screens in excluding insects from the greenhouse to be quantified. This allows a comparison between screens, and therefore selecting the most efficient giving maximum insect exclusion with maximum porosity.