L. H. Godinho1*, M. P.F. Gra^a 1,2

1) Prirev, Lda, Zona Industrial de Vagos, Lt.61, 3840-385 Vagos — Portugal
2) Physics Department, I3N — Institute of Nanostructures, Nanomodelling and Nanofabrication,
Aveiro University, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal
* Corresponding Author, lhalgodinho@gmail. com

Abstract

The main purpose of a greenhouse is to create and maintain a controlled artificial environment that will favour the crop production with the maximum profit. Late increase on fuel prices, together with colder than normal seasons, make heating costs a significant burden on greenhouse operations. Therefore, the use of renewed energy systems, namely solar thermal systems, to control the inner environment of agricultural greenhouses becomes an economical and technological topic of unquestionable interest. This publication is related with the analytical analysis of the possibility of using a solar thermal system to control the climate environment of a greenhouse. Preliminary calculations show that, in certain climate conditions, a solar greenhouse can collect sufficient solar energy to feed, at least, another standard thermally optimised greenhouse of the same size. The implementation of this project, which is based on Portuguese Utility Model n°10218 — “Thermal Solar System for Collection, Storage and Distribution of Heat at Low Temperature”, considers the construction of a prototype and, later, of a larger industrial greenhouse, to verify the technological and economical viability of the patented idea.

Keywords: greenhouse, energy, solar, agriculture,

1. Introduction

Since the beginning of the XX century the world has been suffering from rising exploitation of its natural resources, with the resulting consequences in pollution and degradation. Oil, for example, considered a traditional energy source, have been extracted in such huge amounts that oil-wells already started to be depleted less than 100 years after the beginning of its effective use.

The “Lisbon strategy”, set in March 2000 by the European leaders, assumed the commitment of the UE to become, up to 2010, an economy based on the most dynamic and competitive knowledge of the world, capable to guarantee a sustainable economic growth, with more and better jobs, bigger social cohesion and respect for the environment. One of the main goals will be the ambient sustainability, that is, to develop and to spread out the echo-innovations and to build the leadership in the echo-industry; to adopt policies to generate long term improvements supported in increased productivity through echo-efficiency. Economic growth will be supported by the echo-innovations leading to a decrease in pollution and to a more efficient management of the resources. Many examples of these echo-efficient innovations already exist in several areas, agriculture and energy included.

The conversion of traditional agriculture, which is strongly climate dependent and characterized by extremely hard work conditions, in a more technical agriculture in controlled environment, will have an enormous impact in the reduction of the risk and in the increase of productivity and quality of the cultures, providing more appealing work conditions.

Basically, greenhouses are solar collectors with poor heat storage capacity. From local weather conditions in the last years we know that in late autumn, winter and beginning of spring, night temperatures can be quite low for long periods. Temperature represents a critical factor to the survival and growth of the plants. In fact, keeping the greenhouse at the most suitable temperature for the development of a given plant will significantly improve its productivity and quality.

Also in summer we can have in greenhouses a phenomena called “inversion” whenever the inside temperature drops dramatically at dawn when the sun rises, due to the evaporation of water condensed during the night.

Traditional Portuguese greenhouse consists of a galvanized iron pipe structure, covered with single layer of polyethylene (PE) film. This type of greenhouse implies enormous heat losses with consequent high fuel consumption. As a result, an increasing number of farmers have just switched off the heating and given up some greenhouse cultures in the last years due to the rise in energy costs.

The main objectives of this project are the thermal efficiency analysis of the greenhouse (type of covering, heating system, thermal curtains, etc.) and how much of its heating by traditional energy sources can be replaced by solar energy. The economic viability of its construction, the use of low cost (and yet efficient) polymeric solar thermal collectors together with simple water heat storage tanks — made from expanded polystyrene (EPS) blocks with an inner bag of reinforced polyvinyl chloride (PVC) film — and the way to transport and release the thermal energy inside the greenhouse, will also be studied.

During the cold night periods, water from the storage tanks will flow through small polymeric tubes inside the greenhouse and will heat the ground (if buried), or the bottom of the vases (if covering the growing tables).

At the energy level, the phases of this project are:

• pre-study of thermal properties in the design phase of the project;

• design of energy monitoring systems;

• data analysis and presentation of results.

Intended results are:

• substantially improved production conditions (quantity and quality);

• increased effectiveness of the heating using thermal efficient greenhouses;

• lower fuel consumption and, consequently, less air pollution;

• promotion of specialized agriculture, more competitive and with better work conditions.

In this paper we describe the analytical analysis that supports this solar thermal system project.