Solar air heating system with low environmental impact materials: Mathematical model and optothermal characterization
Graphical abstract
Introduction
The residential sector requires basic technologies to meet the demand for services such as water heating, cooking, illumination, mobility, and home heating [1]. Heating living spaces is a necessity of populations living in areas with cold climates and it is important to pay special attention to the types of technologies involved in the generation of this heating [2]. In some countries like Mexico, home heating is done via thermal insulation or by using different technologies for this purpose [3], [4], these heating technologies are important elements within homes because they allow generating thermal comfort and social well-being [5]. However, the aspects of affordability, access, and environmental impact are not always taken into account in the development of these technologies. People usually buy heaters that are expensive, sophisticated, and made of high environmental impact materials; therefore, ecotechnological alternatives are required to address the heating problem in the residential sector [6]. On the other hand, in the industrial sector, the use of efficient heating systems makes it possible to optimize processes, reduce costs and take advantage of renewable energy sources.
Heating systems with renewable energy sources are increasingly utilized, some are efficient and have been used in different applications on small and large scales, individually or in combination with other technologies. Some of these systems are based on the use of residual heat and energy efficiency [7], while others for example, like the work of Ghorbani et al. [8], have incorporated a system of flat solar collectors to supply heating in a natural gas plant. Mehrpooya et al. [9], have also used solar collectors for fluid heating in compound operations, in their case for the generation of biogas. Some other processes have incorporated combined renewable sources, such as the plant described by Ghorbani et al. [10], that incorporates photovoltaic systems and geothermal energy for the supply of energy and heating to produce bio-liquefied gas. Chahartaghi et al. [11], have shown a geothermal heat pump for heating fluids in the residential sector. Phase Change Materials (PCM) are also used efficiently in systems composed of several renewable energy sources for the production of cryogenic energy, such as the one developed by Ghorbani et al. [12]. There are also solar heating systems that use the thermosyphon effect [13] which can also be used in combination for power generation and cooling processes. Huang et al. [14] used a PCM for solar heating systems, to increase the thermal storage capacity which allows to show the thermal viability of these materials in heating systems. Although studies by Abdelsalam et al. [15], show the viability in heating needs of a home by adding PCM to a thermosyphon cycle system, the costs can be high, at the present time. In this sense, thermochemical heat storage systems (THS) have important advantages over other thermal storage systems, in particular a high energy density and low heat loss when hermetically sealed. [16]. Additionally, Alva et al. [17] present an interesting review of the generation of solar energy, the thermal comfort of buildings and other specific applications of Thermal Energy Storage (TES); in these systems the materials are important because they can include some of low environmental impact, for example the TES that use limestone, reported by Zhangke et al [18], and Sarrión et al. [19] or those shown by Durán-Martín et al. [20] that carry out structural characterization of materials, due to their functionality as efficient thermal accumulators; although many TES turn out to be sophisticated and they are commonly also expensive, but their low environmental impact contributes to the generation of sustainable technologies, where the environmental impact is sought to be minimal, as shown by Di-Florio, et al. [21].
In the residential sector, green air heating strategies are diverse, some use renewable energy sources and sustainable materials, others are passive or dynamic. A passive thermal conditioning system can range from strategically-placed windows, air dehumidifiers and ventilation towers, to green roofs. For example, some passive systems are of the wall type and include Trombe walls [22]. Additionally, systems that use PCMs have acquired importance in recent years [23], and have also been used in low-power eco-technologies [24]. The development and implementation of alternative thermal conditioning systems based on the exploitation of solar energy have also increased. These are called thermosolar systems because they capture radiant solar energy and use it for air or water heating. In these solar devices, energy and fluids are mobilized using energy gradients produced through the absorption of radiation [25]. Thermosolar systems may or may not be equipped with a thermal reservoir –built with various configurations of materials– to store heat [25], for this purpose, one recommendation is to use materials with a high thermal capacity to store calorific energy in a reservoir. One important advantage of these technologies is that they are easy to replicate. Some researchers have shown interest in materials for thermal reservoirs like rocks [26] and sand because they are readily available [27]; for example, using gravel to accumulate energy and generate distillation[28], or heat fluids [29], some mixtures of sand and phase change material for technologies high-performance thermal energy storage [30], to improve the performance of hemispherical collectors [31] or desert sand for high-efficiency solar collectors [32]. Furthermore, these technologies allow the construction of efficient devices [33], are of low cost, and have a low environmental impact.
Sophisticated Heating systems such as thermoelectric systems [34], systems combined with renewable energy sources, optimization systems, multipurpose systems, or simple systems with low-cost local materials; can generate multiple tasks. One of the main motivations of this work is to generate a low-cost solar ecotechnology for air heating with low environmental impact local materials that is easy to reproduce in families of vulnerable groups and also is not sophisticated and has the necessary efficiency to achieve the heating of small spaces where people from rural or indigenous communities live. Furthermore, in the case of systems that use renewable energy sources, the exergy efficiency process is essential because it analyzes the quantity and quality of the useful exergy. The concept of exergy analysis is very important for the optimal use of energy in any system as well as for planning the design and processes of industrial operations [25]. The exergy efficiency is defined as the ratio between the exergy gained by the fluid and the exergy gained by the system; and in thermosolar systems, the fluid heating process is usually efficient [25]. In Europe, works have been carried out where the cost of solar heating systems is calculated [35], this represents a task that should also be carried out in Mexico, since in this case low cost is a necessary condition. The study of these systems is of special interest for countries like Mexico, where many localities have low-temperature climates, and therefore require to heat spaces [3].
This research shows the development of a solar air heating system that uses limestone, gravel, and soot forest biomass as the functional elements for increasing solar absorption and thermal accumulation; this device has been evaluated to determine the optical and thermal properties that will provide the optimal ratio among the materials involved to make the technology sustainable and optimized for heating processes in interiors and other applications. This research aims to show a photothermal composite material for thermal storage not reported in the literature, from local materials, with low cost and low environmental impact. A mathematical model has also been developed that allows to know the behavior of the system and helps to optimize the application of photothermal materials used in this technology.
Section snippets
Materials and methods
This research proposes the use of gravel, limestone, and soot forest biomass in various concentrations to develop a solar device for air heating. The combination of these components is called Thermal Accumulation Material (TAM):
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Prototype TAM 1 (T1): 95% gravel-5% soot
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Prototype TAM 2 (T2): 95% limestone-5% soot
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Prototype TAM 3 (T3): 50% gravel-50% limestone
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Prototype TAM 4 (T4): 45% gravel-45% limestone-10% soot
The objectives in varying the ratio of these components are: (a) demonstrate the
Results and discussion
The design of the device consisted in a tube for conducting and heating air that passed through the TAM and then re-directed the air to its destination (Fig. 4a). After the mechanical grinding process, the different concentrations of the TAMs were placed in the wooden box under the glass cover and solar collector. Fig. 4b presents a flow diagram to illustrate air circulation during the heating process and the elements included in the system. To insulate as much as possible, the wooden box where
Conclusions
This research showed the development of a solar air heating system, considering a thermal accumulation reservoir composed of different concentrations of limestone-gravel and forest biomass soot (TAM). The optothermic properties of the different concentrations were determined and the results indicate that the materials with the presence of gravel and soot showed solar absorptive values above 92%, which is the particular case of the 95% gravel-5% soot concentration, in these composites. Soot acts
CRediT authorship contribution statement
L.B. López-Sosa: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing. A. Ortíz-Carrión: Conceptualization, Methodology, Data curation, Writing - review & editing. D. Espinosa-Gómez: Software, Investigation, Writing - review & editing. J. Zárate Medina: Supervision, Validation. M. González-Avilés: Supervision, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the Universidad Nacional Autónoma de México, the Universidad Intercultural Indígena de Michoacán, and the Universidad Michoacana de San Nicolás de Hidalgo, for their support in the development of this research.
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