Elsevier

Renewable Energy

Volume 103, April 2017, Pages 570-581
Renewable Energy

Analysis of the technical, environmental and economic potential of phase change materials (PCM) for root zone heating in Mediterranean greenhouses

https://doi.org/10.1016/j.renene.2016.11.040Get rights and content

Highlights

  • The thermal behavior of a perlite bag from a Mediterranean soilless protected crop is described.

  • A TES system with PCM to heat plants roots in soilless crops is studied with multiple experiments.

  • The best PCM phase change temperature for the application seems to be 15 °C.

  • The most effective PCM location consists of wrapping the perlite bag.

  • 20–30 Kg of eq. CO2 emissions could be saved per hectare and night.

Abstract

Root zone heating systems offer increasing crops quality and productivity. However, these systems are based on the use of nonrenewable fuels. This paper reports on a study of different design solutions for a root zone heating system, based on thermal energy storage with PCM. The objective of the study was to define, through multiple experiments, the most efficient PCM melting/freezing temperature and location with respect to the substrate (i.e., under the substrate) for the application under study; as well as, to determine the system’s environmental and economic feasibility, with life cycle assessment and life cycle cost methodologies. Results show that the best melting temperature for the application under study is 15 °C. To increase the efficiency of the system, PCMs may be macro encapsulated and wrap the entire perlite bag. Moreover, it seems that PCMs are far to substitute conventional root zone heating systems because it does not provided enough heat during nights. Nevertheless, PCMs can help to reduce the operation time of conventional systems. Based on one night results it seem that PCM could provide annual saving of between 22 and 30 kg of eq. CO2/ha·day. However, it does not seem to be feasible if PCM prices (8€/kg) do not decrease significantly.

Introduction

Thermal energy storage (TES) systems allow the storage of large amounts of cold or heat for long periods of time (hours, days or month) and recover the heat when required. Phase change materials (PCM) have been used to create different TES applications, e.g., for buildings [1], [2], [3], waste heat collectors [4] or storage and transportation cooling systems [5]. PCM are substances with a high heat of fusion and specific melting and solidifying temperatures and can be used for storing and releasing large amounts of thermal energy. PCM can be used to store solar energy during the day and release it at night.

PCM applications are of great interest due to the capacity of PCM to increase systems’ energy efficiency and reduce their dependence on nonrenewable resources. Few PCM applications have been designed to improve greenhouse heating systems [6], [7], [8], [9], [10], [11]. These applications aim to reduce greenhouse heating system energy consumption by (1) using solar collectors inside [6] or outside [9] the greenhouse based on PCM use, (2) installing a ground-source heat pump-phase change material for latent heat storage system in greenhouses [7], (3) increasing the energy efficiency of heat pumps with new PCM applications [8], (4) using PCM to reduce daily temperatures without the use of cooling systems [10], or (5) installing a north wall made of PCM inside the greenhouse as a TES [11].

Providing proper temperatures to the root of crops stimulates plants development and flowers production, which results in an increase of productivity [12]. Yasushi et al. [13] compared the production yield of a tomato crop without a root zone temperature control system and a crop during which nigh root temperatures were maintained over 15 °C by using a root heating system (no air heating was used). The yield of the heated crop was 20% higher and its fruits were 30.5% heavier. Another study [14] detected for tomato crops a decrease of flower production and a reduction of fruit weight when ambient temperatures increase above 25 °C or decrease under 15 °C. Therefore, it seems that tomato plants ideal root and ambient temperatures to enhance productivity may be maintained between 15 °C and 25 °C. Root zone heating can be combined with air heating [15]; nevertheless, heating specifically the root zone can reduce fuel consumption rather than heating the air of the greenhouse [16].

Some authors have looked for the ideal root temperatures for other crops. For example, has already been reported that a proper root temperatures for lettuces production could be between 17 and 24 °C [17]. For the case of maize production no significant effects were detected in plants and productivity if root temperatures are maintained between 13 °C and 28 °C [18]. For other crops such as tobacco, cotton, corn or pea ideal root temperatures may be 32 °C, 25 °C, 20 °C and 10 °C, respectively [19]. Therefore, ideal root temperatures are different for each plan.

Root zone temperature control has been conventionally achieved with expensive and unsustainable gas and oil heating systems. Some PCM systems to control root zone temperature have been studied as a means to improve the environmental performance of crop production systems while ensuring the increase of crop production. An actual experience with PCM for root zone temperature control was found in the literature [20]. The study testes, with positive results, two PCM with a similar phase change temperature of 12 °C in soilless protected crops in Turkey. In this case, PCM were located next to the perlite bags. The manuscript concludes that further research is required to investigate PCMs with other thermal properties and different encapsulation shapes. In addition, they suggest that most of the thermal energy stored by the PCM was lost by convection to the air at night, fact that reduced the efficiency of the system.

Another study analyses the environmental and economic feasibility of a theoretical application of PCM, in a greenhouse with 19.4 ha from southern Spain, as a substitute to conventional root zone heating systems which use oil, gas or biomass boilers [21]. This study concludes that PCM seem to be a potential technology to substitute conventional heating systems and provide environmental and economic benefits. Nevertheless, it is based on detailed calculations and not real data. For this reason the manuscript concludes that there is the need generate and use real data to reduce the uncertainty of its results.

Some studies have concluded that soilless culture is the most intensive and effective production system in the agricultural industry [22], [23]. Soilless culture is based on systems which allow plant growth without the use of soil as growing medium. Plants can be grown in porous substrates or directly in contact with water without the use of substrates. In soilless crops the exact nutrients required by plants are mixed supplied through the irrigation water [24], making these crops efficient in terms of water and nutrients per kg of production.

Substrates used in soilless culture can be divided in two groups: organic (i.e. coconut fibers, wood residues, bark, rice hulls) or inorganics (roockwool, sand, perlite, pumice). Inorganics have the advantage of being free of potential diseases, pests and weed seeds. Moreover, they drain better than organics, fact that allow a better control of soil conditions (i.e. nutrients content, available water for plants) [25].

In Spain and the Mediterranean area, it is common the use of perlite as a substrate in soilless crops. In fact, in Southern Spain, where is concentrated nearly all greenhouse tomato production in Spain (Spain produced 3.68t of tomato in 2013 [26]), perlite is a common substrate used. Despite the importance of its use and the relevance of root zone temperatures on yield production [12], [13], the thermal behavior of such substrate in protected crops has not been properly defined.

Root heating systems combined for soilless crops with perlite substrates can increase significantly production yield with an efficient use of water and fertilizers. However, it is still required further research to determine the thermal behavior of perlite bags and how root zone heating systems can be environmentally improved with the use of PCM. For these reasons, this essay seeks to (1) describe the thermal behavior of a conventional perlite bag which has not been sufficiently described in the literature; (2) select the most effective position for the PCM in relation to the cropping bag (i.e., under, next to, wrapping the bag) to minimize root zone temperature decrease at night; (3) determine the best PCM melting/freezing temperature for its application in Mediterranean greenhouses; and (4) quantify the carbon footprint and economic savings that PCM could provide over conventional gas, oil or biomass root zone heating systems.

Section snippets

Experimental area and crop

The study was completed in a greenhouse situated in Cabrils, north Barcelona (Latitude: 41° 31′ 2.6″N, Longitude: 2° 22′ 39.3″E) under a Mediterranean climate. The greenhouse was 19.2 m wide and 12 m long, with a 3 m high gutter and a 5.5 m high ridge, covered with a single PE layer opaque to far infrared radiation. Average, minimum and maximum temperatures for each season during 2014 in Cabrils are provided in Table 1 [27]. In this region, average and minimum temperatures between November and

Thermal behavior of a conventional perlite bag

The results are shown in two separate graphs (Fig. 3) by grouping results from thermistors situated in axis x (T-1; T-2; T3) and y (T-2; T-4; T-5) according to locations defined in Fig. 2. In addition, Fig. 4 shows a scheme of the daily temperature pattern based on the experimental measurements.

As shown in Fig. 3, Fig. 4, the temperatures in the perlite bag before sunrise were similar at the bottom and the center but slightly higher at the bottom (0.5 °C). The external parts of the bag (top,

Conclusions

  • An appropriate melting and freezing temperature for a root zone passive heating system with PCM in Mediterranean greenhouses seems to be 15 °C. A melting/freezing temperature of 12 °C does not ensure the freezing of the PCM if temperatures do not fall under 10 °C.

  • The best PCM location for the application under study may be wrapping the perlite bag with the material to insulate it from air temperatures.

  • PCM could be used to increase the thermal efficiency of conventional heating systems but PCM

Acknowledgements

The authors thank the Spanish “Ministerio de Economía y Competitividad” (MINECO) for financial support to the research project “Agrourban sustainability through rooftop greenhouse’s, Ecoinnovation on residual flows of energy, water and CO2 for food production” (CTM2013-47067-C2-1-R), the INIA project number RTA2012-00039-C02-01 and the Catalan Government, La Generalitat de Catalunya, for awarding a research scholarship (FI-AGUAR 2015) to Pere Llorach Massana.

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