**1. Introduction**

Social and economic development, together with an increase in human welfare, has led to an increase in energy consumption. Energy services in societies become essential to satisfy the demands of light, heating, transport, communication, etc., as well as for the manufacture of goods. Fossil fuels have been the main source of energy since 1850, and since then there has been a growing demand worldwide [1]. This has led to a progressive increase in the levels of carbon dioxide in the atmosphere, reaching a record 415.70 ppm in May 2019 [2]. Such a concentration of CO2 in the Earth's atmosphere had not been reached for more than three million years, when the global sea level was a few meters higher and Antarctica was partially covered with forests.

The provision of energy services to a world population in continuous growth has increased the emission of greenhouse gases (GHG), these being the main cause of global warming. In December 2015, the Paris Climate Change Conference, also known as COP21, was held in Paris. It established the world's first binding climate agreement, through which the 195 signatory countries establish a global action plan to keep global warming below 2 ◦C between 2020 and 2030, and below 1.5 ◦C if possible [3].

In order to tackle global warming, two types of strategies have been developed: those for mitigating climate change and those for adapting to it. Mitigation strategies aim to reduce greenhouse gas emissions in such a way that the point of no return is not reached, while adaptation strategies aim to reduce the consequences of an already apparent climate change and address its impact.

Climate change mitigation measures that can be taken to reduce pollutant emissions include increased energy e fficiency, greater use of green energy within the global energy mix, electrification of industrial processes, promotion of more sustainable transport (electric mobility, cycling) or higher carbon taxes as well as a market for CO2 emissions [4]. Therefore, it can be said that climate change is a global problem demanding a shift from the current energy model [5].

Measures to adapt to climate change that can be taken in order to reduce its e ffects include the construction of adapted housing and workplaces, reforestation, adaptation of crops to new agronomic variables, development of emergency plans to deal with possible natural disasters, or research into possible e ffects on human health [6].

Renewable energies have demonstrated their potential in mitigating climate change, but they also have other benefits. Such forms of energy favour local employment and thus contribute to economic and social development, facilitate access to energy, increase energy resilience, and reduce pollution in cities, and therefore the e ffects on human health [7]. The concept of renewable energy encompasses heterogeneous categories of technologies. Some renewable energy sources can provide electricity, others supply thermal or mechanical energy, and others can provide biofuels to meet various energy demands. Some renewable energy technologies can be adopted at the point of consumption (centralized) in rural and urban environments, while others are implemented mainly in large supply networks (decentralized). Although more and more technically advanced renewable energy technologies have been adopted on a medium scale, others are at a less advanced stage and have a more incipient commercial presence or supply specialised market niches.

Within renewable energies, bioenergy has undergone a grea<sup>t</sup> development in recent years. This is due to its carbon neutral balance and the fact that bioenergy can be obtained from a range of biomass resources, including residues from forestry, agricultural, or livestock industries, the rapid rotation of forest plantations, the development of energy crops, organic matter from urban solid waste, and other sources of organic waste from agro-food industries [8]. These residues can be burnt directly to produce electricity or heat or can be transformed by using physicochemical processes to generate gaseous, liquid or solid fuels. Bioenergy technologies are very diverse, and their degree of technical sophistication di ffers considerably. Some already marketed are small or large boilers, district heating systems, or the production of ethanol from sugar and starch. Bioenergy technologies have therefore applications in both, centralized and decentralized contexts, and their most widespread application is conventional use of biomass in industrialized countries for heating and power generation [9]. Bioenergy production is often constant or controllable. Bioenergy projects generally depend on locally and regionally available fuel, although recently there seem to be indications that solid biomass and liquid biofuels are increasingly present in international trade. Within centralized context and small-scale systems, biomass boilers are an emerging technology with numerous economic and environmental advantages [10].

Traditionally biomass boilers have been powered by pellets composed mainly of chips and sawdust from the wood industry [11]. Due to the increasing trend in the prices of biomass from the forest and wood industry, it has been necessary to investigate other sources of bioenergy, which allow lower costs while reducing the pressure on the agroforestry sector. Many investigations have shown the potential of certain fruit stones and nutshells in the generation of thermal energy at industrial and residential levels [12,13]. It is therefore necessary to find alternative biofuels as enacted by the European standard EN 14961-1.

*Eriobotrya japonica*, commonly called Japanese loquat, or simply loquat is a perennial fruit tree of the Rosaceae family, originating in south-eastern China, where it is known as "pi ba". It was introduced in Japan, where it was naturalized and has been cultivated for more than a thousand years. It was also naturalized in India, the Mediterranean Basin, Canary Islands, Pakistan, Argentina and many other areas. Today, Japanese loquat cultivation has spread throughout the world, both for its ornamental value and for its prized fruits. The loquat is cultivated mainly in China, Japan, India, Pakistan, Mediterranean countries (Spain, Portugal, Turkey, Italy, Greece, Israel), United States (California and Florida), Brazil, Venezuela, and Australia. According to data provided by the Food and Agriculture Organization (FAO), China, in addition to be its country of origin, is the world's largest producer. In the last ten years China has doubled its area of cultivation and production, reaching 118,270 ha. and 453,600 Tn per year [14]. Spain is the world's second largest producer and exporter of loquat with an annual production of around 30,000 tn. Andalusia has around 1,100 hectares of loquat spread between Granada (815 hectares) and Malaga (275 hectares) [15]. The consumption of this tropical fruit has gradually increased over the last two decades and a good performance of demand is forecast for the coming years. Figure 1 shows the loquat production in the main countries in the world.

**Figure 1.** Loquat production in the main countries [16].

Processing factories that use loquats to make products such as liqueurs and jams generate large amounts of waste mainly in the form of skin and stones or seeds. Inside the loquat contains between 3 and 7 large brown seeds representing between 15% and 18% of the total weight of the fruit [17]. These wastes are disposed of and sent to landfills without making environmentally sustainable use of them.

The water in heated swimming pools needs an external supply of energy in order to maintain thermal comfort, as the natural tendency of the water will be to equalize the temperature of its environment. If the temperature of your environment is lower, the temperature of the water will decrease depending on the conditions of the environment such as: temperature of the air, of the walls and floor of the pool, etc. As a first consideration, we must bear in mind that the energy consumption of this type of installation can be higher than 700 MWh per year, depending on the climate of the place where it is located, the conditions and seasonality of the use, and the demand required by the systems included. Therefore, a small improvement in energy e fficiency for proper operation translates into a big saving in global terms in CO2 emissions to the atmosphere and the use of scarce energy resources. It should be noted that not only is the demand very high, but in most cases, there is a priori ignorance of the amount of energy that will demand a heated pool [18–20].

The main objective of this study is to determine, on the one hand, the energetic properties of the loquat seed and, on the other hand, to evaluate its suitability to be used as a solid biofuel to feed the boilers of the heated swimming pool of the University of Almeria (Spain), highlighting the significant energy, economical, and environmental savings obtained, contributing to reduce greenhouses gases.

#### **2. Case Study**

As a case study has been taken a university sport centre located in the University of Almería (UAL), Southern Spain (Figure 2).

**Figure 2.** Location of University of Almeria.

The UAL Sports Centre is made up of indoor spaces in which different sports can be practised: sports centre pavilion (central court and two lateral courts), rocodrome, fitness room, cycle inner room and indoor swimming pool.

The water volume of the heated swimming pool is 494.15 m3, has a compensation vessel of 30 m<sup>3</sup> and has an internal dimension of the water sheet of 12.5 × 25.10 m. Figure 3 shows the indoor swimming pool of the UAL Sports Centre.

**Figure 3.** Indoor swimming pool of the University of Almeria.

Firstly, an energy audit of the installation was carried out to establish a technical basis for this study. In addition, all the necessary data was collected to study the possibility of replacing the existing boiler with a biomass boiler using loquat seed as biofuel.

#### *2.1. Meteorological Data*

Almeria has a Mediterranean climate, with mild winters, hot summers, and little rain. The average annual temperature is 17.9 ◦C and the average rainfall is 228 mm. Table 1 shows the most important meteorological data of Almeria.


**Table 1.** Meteorological conditions in Almeria.

#### *2.2. Description of Existing Thermal Facilities*

When planning the heating of an indoor swimming pool, some fundamental differences must be considered compared to a residential building heating system: firstly, there is a high level of evaporation on the premises, and secondly, the comfort conditions for bathers are different.

According to current regulations, the temperature and relative humidity of the room must be adequate to protect the health of the users (RITE, Complementary Technical Instruction) [21]. As for the temperature of the ambient air, the water temperature, and the environmental humidity, the following were taken as comfort conditions:


At present, the indoor swimming pool of the University of Almeria has two fuel oil boilers, with a nominal power of 267 kW. One of them is a backup in case of failure.

The current installation has the following auxiliary elements:


**Figure 4.** Boiler room of the indoor swimming pool.

This installation has an annual consumption of 52,239 liters, as Table 2 shows.


**Table 2.** Annual consumption of the indoor swimming pool boiler for the year 2018.

In Spain, Royal Decree 742/2013 [22] classifies swimming pools with respect to public access and classifies them into public and private use swimming pools. In relation to the temperature of the air and of the bath water, heated or covered swimming pools are defined as those in which the enclosure where the glasses are located is closed, has a fixed structure, and the water is kept at a more or less hot temperature.

Figure 5 shows the operating diagram of the fuel oil boiler.

**Figure 5.** Operating diagram of the fuel boiler..

#### **3. Materials and Methods**

Regarding the use as biofuel of loquat seeds from agro-food industries, 2000 g of such seeds were collected for their subsequent energy and chemical composition analysis (Figure 6).

**Figure 6.** Eriobotrya japonica, commonly called Japanese loquat.

UNE-EN 14961-1 standard "Solid biofuels–Specifications and fuel classes—Part 1: General requirements", were used to assign the biomass quality parameters. This standard has been developed by the Spanish Association for Standardisation and Certification (AENOR).

Table 3 shows the standards and the measuring equipment used.


**Table 3.** Standards and the measuring equipment used in this study.
