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detailed review of the way in which solar thermal and photovoltaic electricity should be used and the incidence of this solar energy in the calculation of the thermal load.
by: Camilo Botero*
The use of alternative energies is gaining strength more and more, however, this issue still accounts for a very small percentage of the cake in terms of its use by the air conditioning and refrigeration industry. Below we will break down the issue of solar radiation as an alternative energy and highlight the efforts of our sector to massify this option.
Definition of radiation: Thermal Radiation: it is cataloged as one of the many existing electromagnetic phenomena. It is due to the temperature that a body has. It can then be said that radiation is energy emitted by the body in the form of electromagnetic waves by virtue of its temperature. As is known from physics, the amount of energy emitted is a function of the wavelength, it is between 0.1 and 100 μ.
For a given temperature, the total maximum energy emitted, at all wavelengths is: 
If A1 were a gray body; that is, if its energy emission is a constant fraction of the maximum energy emitted.
Now, if it is two gray bodies, which have a certain position one with respect to the other the heat exchange is given by:
Sometimes you want to put the heat flow, in terms of the linear difference in temperatures, for this, the average coefficient of heat transfer by radiation is defined:
I will focus this article on two parts: On the one hand the generation of solar thermal and photovoltaic electricity, and on the other the incidence of this solar energy in the calculation of the thermal load, which is extremely high in tropical countries.
The book that appears attached, has been tremendously motivating for me that in Colombia and other Latin American countries, in most of which there is a great intensity of solar radiation we concentrate (among others), on the use of radiation as renewable energy.
Solar thermal electricity, lives a moment of great interest in the concert of nations, finally its takeoff has occurred and our Ibero-American countries can not be oblivious to this challenge that represents a great opportunity to introduce ourselves in a significant way in the use of this renewable energy. Spain already gives us the example categorically since it has been one of the pioneer nations, even though now for reasons of its economic crisis, it has neglected this issue.
Institutional support for these developments must be resolved as soon as possible, involving scientific, academic and governmental institutions with their knowledge resources and above all financial support for initiatives, since solar thermal energy will undoubtedly have a very important role in the energy system of the future, which will surely be more decentralized and diversified, for example, it is being used in oil wells to generate the steam required by the process, thus being able to commercialize the gas that was used for that purpose. This will make the rational use of energy more efficient and supportive. In Colombia it would be a great alternative for the interconnected system that when there are droughts must use conventional thermal plants of very low efficiency, of great capacity of deterioration of the environment and factor of climate change. The use of solar radiation as a resource to heat domestic water and cook, is obvious and surprising precisely because of the obvious that we do not use it on a large scale.
Some considerations regarding Colombia regarding the use of solar radiation: The UPME (Energy Mining Planning Unit), in partnership with the Institute of Environmental Studies - IDEAM, published the Atlas of solar radiation, which is the official document of this energy resource in the country. There are presented, registered, the values of the energy resource for the development of solar energy systems in Colombia. In this work, measurements with high values of solar radiation are recorded, which indicates the great energy potential by solar radiation, existing in some regions of the country.
This annual solar radiation map shows that the isolated zones or Non-Interconnected Zones to the system, present average values of 4 - 4.5 kWh / m2 of radiation per day, which is suitable and very motivating for the development of new programs of application of photovoltaic systems, given that in countries where photovoltaic solar energy has been developed, similar radiation values are used. In the case of southern Spain with the highest radiation and the largest solar plants and platforms in that country, an average daily solar radiation of 3.8 to 4.5 kWh/ m2 per day is recorded.
The country is characterized by the regions: Andean, Atlantic, Pacific, Orinoquia and Amazonia. The intensity of solar radiation in the Colombian Andean region shows that the areas of Valle del Cauca and Magdalena have the greatest potential in this region, and as you ascend to the tops of the mountain ranges that potential gradually decreases, with the exception of some areas called altiplanos, where a greater solar potential is observed compared to that of the slopes and valleys.
Measurements made at another point in Valle del Cauca contrast with the values obtained from the official document. This is how in Lake Calima, municipality of Darién, in Valle del Cauca, where there are no known weather stations that report solar radiation, with a digital pyrometer and solar radiation values have been measured, which exceed 1000 W / m2
As for values of photovoltaic solar energy, values higher than 1200 Wm2 were measured at the Autonomous University of the West in Cali, which is quite high and promising for photovoltaic generation, said university has already installed a photovoltaic electric power generation system with a capacity of 140 KVA. At ciat in Palmira, where a germplasm laboratory for beans and cassava is being designed, with the concept of Zero Energy Balance (NZEB, first in Colombia), all the necessary energy will be generated, with photovoltaic cells in an area greater than two hectares.
As an excellent example of the impact of solar radiation, we can present the competition called Solar Decathlon (www.solardecathlon2015.com.co), which took place in Cali Colombia, from December 4 to 15, 2015, for the first time in Latin America; in which interdisciplinary groups belonging to universities around the world, work on the design and construction of sustainable housing, in their countries of origin. The selected teams moved their homes to Cali, where they assembled them in a time of 10 days, which constitutes a great challenge of planning, coordination, engineering knowledge in various disciplines. The Solar Decathlon was created in 2002 by the United States Department of Energy and since that year, it has been carried out in 2005, 2007, 2009, 2011, 2013, 2014, 2015; it has taken place in the United States, Europe, Asia and now in Latin America.
In this way the Villa Solar was built at the Universidad del Valle in Cali, in an event open to the public and as a relevant fact attended by around 80,000 people, which motivated the organizers of this competition to select Cali again, for the next competition, for having been a record attendance of individuals, and more than 600 decathletes, who built the houses.
Also fortunately for the competition, every day was extremely sunny, with intensities of solar radiation of the order of 1000 W / m2, which highlighted how important is the use of solar energy in homes, in these tropical climates. All the houses generated more energy than they consumed and these PV generations were connected to the EMCALI network, which is the city's electric power company and this entity, supplied at no cost obviously; the energy for the needs during the night (mainly lighting) and during the construction of the Solar Villa, since the houses did not have accumulators.
There was also a site called the Agora, in which lectures were given related to the concepts of sustainable architecture and engineering that were used in the designs and construction of the different houses. During the visits to the houses the young people of the team of each house gave the attendees in groups of 8 people or less, the explanations and details of their proposal, with great enthusiasm and knowledge of what they had done, because in addition to designers, they were the builders with their own effort, which gave them a great sense of belonging.
The evaluation to award the prizes, was made on the following aspects, for a low-cost housing, it is called Decathlon, because the teams compete in ten tests that are briefly described below:
- Architecture; Evaluates appropriate spaces and materials for sustainability strategies, functionality and furniture.
- Engineering and Construction; Evaluation of the structural design, plumbing electrical installations and of course the solar energy system for both PV generation and hot water.
- Energy efficiency; Evaluates designs for the reduction of energy consumption, using mainly solar energy.
- Energy consumption; Energy self-sufficiency is measured, through a balance between generation and electricity consumption, with two-dimensional meters to quantify own generation and/or network consumption.
- Comfort; Temperature, relative humidity, acoustics, lighting and air quality are measured, which are indicators that show the comfort in each home. Most had ventilation-only solutions, either natural or forced. Some incorporated direct expansion air conditioning systems, with PV power.
- Sustainability; It contemplates the reduction of environmental impact and the strategies of architectural designs and engineering, energy efficiency and affordability. Also the management of rainwater, and treatment of wastewater for irrigation or consumption in toilets. Several had built-in orchards, and green walls and ceilings.
- Operation; It measures the efficiency and functionality of a set of appliances such as stove, refrigerator, microwave, washing machine, blender, television, sound equipment, to ensure the normal operation of the house and lighting, all operated with solar PV energy.
- Marketing, Communications; Evaluates the marketing and communications strategies implemented by each team, with the aim of generating social awareness.
- Urban Design and Feasibility; Encourage the implementation of an Urban Design in the context of the region. Most of the houses, especially the national ones, obviously incorporated local materials of low or moderate cost and ease of acquisition.
- Innovation; It estimates the incorporation of creative solutions to improve the residential sector, in the aspects described above.
These initiatives of energy-self-sufficient, low-cost, environmentally friendly constructions are perfectly aligned with the orientation regarding the care of our common home as described by Pope Francis in his encyclical Laudato Si and the agreements signed at the Climate Change Summit in Paris, regarding which Colombia has already committed to the reduction of 20% of greenhouse gas emissions.
Impact of solar radiation in calculation of thermal load
When the calculation of the thermal load is carried out in order to determine what would be the size of the equipment in Tons of Refrigeration, the transfer of heat by solar radiation, constitutes a preponderant factor especially if there are large domes and windows, which are exposed to said radiation.
The information that comes in the American or European texts and/or software and in the excellent manuals of the main equipment firms regarding the intensity of solar radiation in our region is not correct and even though we already have local information, it is not presented in the way that is required in a process of calculating the thermal load to calculate the radiation at peak hours; we must work on organizing this information from our guilds,
What is evident is that the charge by solar radiation through translucent materials in a tropical country is determinant of the capacity of the central systems, with the aggravating factor in some cases that the temperature condition can be met, but because it is a phenomenon of electromagnetic waves it causes thermal discomfort in a very marked way, if the adequate shade is not provided, therefore the reduction of the load by solar radiation is mandatory working together with the architects, issues such as the orientation of the building, thermal insulation and the attenuations of said solar radiation.
* Camilo Botero is the current Secretary of the Federation of Ibero-American Associations of Air Conditioning and Refrigeration - FAIAR; he was president of ACAIRE and is president of Camilo Botero Ingenieros Consultores Ltda. He has worked as a teacher in several Colombian universities, guilds and currently in ACAIRE in diploma courses of air conditioning projects, energy efficiency in air conditioning and refrigeration, cogeneration and trigeneration, applied psychometrics, thermodynamics, fluid mechanics, heat transfer and turbomachinery. ([email protected]).
