Water deficit along with the need of environmental protection coupled with ever-increasing civilization progress forces us to save energy, water and materials [1
]. Results of scientific research and up-to-date industrial achievements in the area of the development [5
] and implementation of intelligent energy systems based on renewable energy indicates that the use of primary energy may be important for achieving national and international goals [9
Intelligent energy management systems are becoming increasingly popular [13
]. Their attractiveness is due to the innovative perception of the energy sector, including issues related to monitoring, controlling, and collecting data. The purpose of their use is communication between all participants of the energy market, with the task of providing energy services, ensuring reduction of costs, and increase of efficiency as well as integration of distributed energy sources, including renewable energy. Intelligent networks give the opportunity to dynamize the development of energy technologies [15
]. The smart systems integrate various resources and technologies in both generation and consumption.
Intelligent energy systems focus on an integrated inclusion of several sectors (electricity, heating, cooling) and allow for the identification of more achievable and affordable solutions in the transition to future renewable [17
] and sustainable energy solutions [20
Hybrid systems, which may include not only renewable sources but also conventional ones, are becoming increasingly popular [22
]. Pioneering projects based on integrated energy systems using photovoltaic installations, heat pumps, and wastewater heat recovery are being implemented [23
]. When waste heat is used, it is necessary to identify its sources and available resources [25
]. When designing a wastewater heat recovery plant, the temperature and quantity of wastewater must be taken into account and it must be decided whether the process will be carried out before or after treatment with substances of a biogenic nature [28
About 1 billion m3
of wastewater water is recovered annually in the EU. As anticipated by EU bodies, the entry into force of the circular economy regulation can increase water recovery from wastewater by up to 6.6 billion m3
by 2025 [31
]. In accordance with the idea of a circular economy, it is assumed that maximizing the use of recycled water in swimming facilities causes maintaining for as long as possible the value of products, materials, and resources, while reducing the generation of waste. Promoting the idea of a circular economy aims at increasing the EU’s competitiveness and protecting companies and, for example, utilities against resource scarcity and price volatility. The reuse of treated wastewater in safe and cost-effective conditions is a valuable but underused way to increase water resources and relieve over-exploited water resources in the EU.
These issues are particularly important for Poland, in which there is three times less water than the average in Europe, i.e., about 1600 m3
per year per person, while in the European Union it is over 4500 m3
]. EU member states have many years of successful experience in reusing water for various purposes. However, it should be emphasized that in Poland, such innovative solutions are only now being implemented and developed.
Considering the dynamics of development and technological advancement as well as the increase in the standard of living and consumer expectations, to ensure energy security, it is necessary to save energy in every possible area of life [33
]. One of the examples of energy saving is the recovery of heat from rinse water in swimming pool facilities.
Regional and local energy systems and networks consist of locally and regionally available energy sources, built infrastructure, as well as user and consumer structures from various sectors. They serve ambitious goals for clean energy for specific communities and regions, as well as for the entire European energy system. They support security of supply, maximizing primary energy efficiency, and ensuring a high share of renewable energy in the energy balance. In Poland, in a situation where in August 2015 there were electricity restrictions caused by too low water levels for cooling commercial power plants with open cooling circuits, it became natural to develop and introduce solutions aimed at saving water and energy [35
Saving electricity, materials, and heat is becoming more and more necessary [36
]. Managers of swimming pools, water parks, hotels, and spas are looking for savings so that while minimizing expenses, they provide greater comfort to users. Maintaining proper water and air quality parameters in these types of facilities require ensuring huge amounts of energy supplied per year, and thus ensuring a budget to cover costs that can and should be reduced. In Poland, often too much emphasis on the purchase price of devices means that they work inefficiently or are simply of low quality. Often, a bad concept of a system that has not been optimized for the best possible ways of energy recovery also has a big impact on bad situations.
In the case of washings arising after cleaning the pool filter beds, the potential is hidden in their large volume and the possibility of using simple unit solutions of processes and devices, e.g., settling tanks, settling tanks, or combined settling tanks with a flocculant mixing chamber. With the appropriate quality of washings, their use for irrigating green areas, sprinkling pitches, tennis courts, and for flushing toilets is a simple solution that allows reducing the costs of water supply and sewage disposal [38
One of the energy problems related to environmental protection is the loss of heat along with wastewater discharged into the sewage system—energy as well as heat is lost, among others in warm washings intended for discharge into public sewers [39
]. Some of this heat can be recovered using a wastewater heat recovery plant.
Energy recovery from wastewater is the subject of many scientific articles. Mostly, the works are concerned with energy recovery from municipal sewers [43
]. Some works are concerned with energy recovery from residential buildings of various sizes [45
] and also various industrial facilities [50
]. A small number of papers deal with energy recovery from wastewater generated by sports facilities, including swimming pools. As an example, the works [51
] give analysis of several Norwegian swimming facilities, coming to the conclusion that in each there is still room for improvement. The other paper [53
] claims that the amount of recovered heat is much higher than energy consumption needed for powering the installation.
The literature contains many detailed analyses from the energy and economic point of view concerning the possibility of optimizing the energy system efficiency of the indoor swimming pool center by means of solar collectors and heat pump technology integrated with the existing installation [55
]. There are also original studies on the long-term operation of the pool heating system through the use of waste heat energy, which is discarded from the ice rink unit [59
]. Energy-saving measures for sports centers of various sizes are analyzed [60
]. In addition, control strategies that can be implemented in the building automation system and the HVAC (heating, ventilation, air conditioning) system of the existing indoor swimming pool complex are analyzed in order to minimize energy consumption [63
]. The energy efficiency in public indoor swimming pools was also analyzed taking into account the concept of passive houses [64
Recovery of heat from sewage is possible and also recommended in places where it is formed in large quantities, including in indoor swimming pools, industrial laundries, etc. which use significant amounts of heat or electricity in their technological processes. Heat recovery from sewage is based on recuperative systems, which consist of heat exchangers or heat exchangers cooperating with a heat pump [65
]. The temperature of municipal sewage from households is in the range of 10–20 °C, while sewage generated in many industries has an even higher temperature—exceeding 40 °C. Lowering the temperature of 1 m3
of wastewater by 1 °C results in 1.16 kWh of heat saving, thus heat recovery from wastewater is justified in terms of energy, environment, and economy [68
In modern facilities, you can now meet various solutions implemented to optimize energy consumption, and thus reduce costs, such as solar panels, ventilation systems with various heat recovery methods, etc. Each of them is specific and should be justified by having a positive impact to manage energy and improve the financial condition of the facility [69
At the global level, there are around 13 million pools, of which around 4.4 million (29%) are in Europe. North America accounts for 59% of the global market, while the rest of the world has only 1.65 million pools (12%) [71
]. According to the European Union of Swimming Pool and Spa (EUSA) associations, France has the most pools in Europe (1.5 million, 34% of the total European number), followed by Spain (27%), Germany (20%), Italy (6%), and Great Britain (5%). In Germany, 250–300 million people visit public swimming pools every year. In Great Britain, 33% of children and 36% of adults visit swimming pools at least once a week, and 55% of children use swimming pools at least once a month. According to available statistics, a large city, such as Paris, has up to 38 public pools, and a public pool can have up to 1400 visitors on a busy Sunday in summer. Public swimming pools therefore have a great social impact, contributing significantly to the health and well-being of the European population, promoting active, healthy, and relaxing recreational activities for all ages and social classes.
In Poland, the total number of indoor swimming pools is estimated at 736, while the number of all pool troughs is estimated at 849. Among this type of sports facility there are Olympic pools, swimming pools, sports-type swimming pools, training, and recreational pools [72
The temperature of the water supplied to the municipal indoor swimming pool is between 5 and 15 °C depending on the season. The analysis of technological possibilities and energy potential has become a premise for attempts to recover heat lost as a result of discharging hot sewage to the sewage system in spot facilities such as a swimming pool. Wastewater generated in indoor swimming pools comes from showers, flushing filters, and discharging pool water. The water temperature in the indoor swimming pool ranges: showers: 38–40 °C, swimming pool: 25–27 °C, recreational pool: 26–30 °C, children’s pool (paddling pool): 28–32 °C, pool for whirlpool: 34–36 °C, rinse water from filters: 25–35 °C. Wastewater temperature is practically constant throughout the year and is in the range of 25–41 °C (on average about 30 °C), so warm sewage in an indoor swimming pool throughout the year can be a constant source of heat [73
]. It also can be observed that demand for heat and electricity is not constant, but undergo variations with maximum consumption during the daytime and sewage release during the night. The time dependences of both processes can be roughly approximated with rectangular shapes shifted with respect to each other.
In heat recovery systems from wastewater, the basic element is the heat exchanger between wastewater and freshwater. Its design should prevent the build-up of impurities such as hair, soap, and others. The presence of impurities could affect the intensity of heat exchange between wastewater and freshwater [76
According to the currently recommended standard (DIN 19645-2006), the rinse water can be treated up to 80% and recycled to the pool water treatment system as freshwater, with drinking water parameters, directly to the overflow tank [78
Sports complexes are an ideal place to enjoy the benefits of renewable energy [79
]. The most commonly used methods are photovoltaic, geothermal and heat recovery from rinsing wastewater. These technologies are an innovative and ecological approach to the use of renewable energy [82
]. Photovoltaic solar farms contribute to the lowering of sports complexes demand for electricity generated outside the facility [83
]. Geothermal energy can be used to heat cold city water that goes to the facility in the same way as the central unit for recovering heat from rinsing sewage [85
]. Both forms reduce the object’s demand for heat generated from conventional energy sources, which are harmful to the environment and their limited amount is constantly depleting on the planet [88
The market has a diverse offer of ready heat recovery equipment for single-family homes, for facilities that consume large amounts of hot water (e.g., laundries, swimming pools, hospitals, etc.) and for sewage treatment plants. On the local scale (heat recovery from the used shower water in the bathroom), there are e.g., ZYPHO (EcoMax, Bydgoszcz, Poland) recuperators using the heat from the used shower water to pre-heat cold water flowing into the battery (with a family of four, you can get 1200 kWh of saved energy per year).
For facilities that consume large amounts of hot water, a heat recovery unit with a recuperator can be used. Wastewater flowing through the internal recuperator coil transfers a significant amount of heat to the freshwater stream flowing through the external recuperator coil. The cold water can be heated up to 30 °C without the use of other heat sources. Another solution is the use (installed in the vertical part of the sewer pipe) of the GFX (Gravity Film Xchange) flow coil. Wastewater flowing down the riser transfers heat to the cold water flowing counter-current inside the copper spiral that wraps the drain pipe. As a result, energy consumption is reduced by more than 7%, which saves 800–2300 kWh per year per apartment. There is also a method of recovering heat from wastewater not directly at the source but in the central collector. With this solution, wastewater flowing into the sewer pipe gives off heat to heat exchangers (built-in pipe segments installed in existing sewer collectors).
The aim of this work is to analyze a case study of an urban sports facility equipped with a modern heat recovery system. The manuscript focuses on demonstrating the feasibility of the investment under the assumed design conditions taking into account economic and environmental aspects. According to the design assumptions, the energy generated as a result of the investment will be used exclusively for the needs of the sports facility. In addition, the manuscript presents additional technological solutions to increase energy savings in sports facilities.
4. Discussion and Research Limitations
The manuscript analyzes variants of potential systems for freshwater heating and sewage heat recovery. Based on the financial possibilities and available technologies, a variant based on the AquaCond heat recovery unit was selected. The amount of heat saved in a sports facility during the analyzed period was the basis for calculating the annual avoided emissions of CO2, SOx, and NOx to the atmosphere when burning fuel or obtaining energy from various sources.
The analyzed sports facility is an urban facility. In accordance with the legal regulations in force in Poland, this type of facility does not and cannot generate any income from the investment, it is a budgetary unit (DNR = 0, DNR—Discounted Net Revenue), the energy produced as a result of the investment is used exclusively for the facility [104
The concept analyzed in the manuscript is based on the synergy of technological solutions. The authors’ assumption was to select possible solutions as a function of financial constraints aimed at increasing energy savings in the sports facility under analysis. The basic concept is based on the recovery of heat from rinse water. Next, the system will be supplemented with a solar kit and modern ventilation systems. Thanks to the modularity of these technologies and their universality, it is possible to build larger investment projects from them.
Sustainable energy policy is most often implemented through the use of renewable energy sources such as solar, wind, and ground energy. The disadvantage of devices such as solar collectors, photovoltaic panels, or wind turbines is that their efficiency depends on changing climate conditions. This disadvantage is not present in the equipment for recovery of waste heat from wastewater generated daily in residential, commercial, or industrial buildings.
Energy in the form of heat, which is generated by other processes and which is not received and used (energy lost irreversibly) is called waste heat. One effective example of its development is cogeneration. In Poland, energy efficiency is about three times lower than in Western European countries [106
]. The need to save energy is an impetus for the use of renewable energy sources and the recovery of waste heat is a response. The problem of using waste heat is very important because many of its sources are unused. New, more efficient, effective, and economical solutions are being sought. The criterion for selecting the method of heat recovery always depends on the specificity of the facility, its geographical location, and investment opportunities. Equally important is the architecture and structure of the building, which should be characterized by low heat losses in the cold period and heat gains in the warm period. There are installations using waste heat from car engines, waste heat from air conditioning, heat from server rooms, waste heat from industry, waste heat from sewage treatment plants, waste heat from the sun. Nevertheless, there are still obstacles to the development of waste heat recovery technologies. Strategically, the fact that the heat used cannot be transported over long distances limits where waste heat can be used.
The recovery of waste heat from sewage in internal sewage systems of buildings requires a dual system in which black sewage will be discharged directly into the external network, while grey sewage, mainly from the shower, bath, washbasin, and sink, will be directed to the heat recovery system. This is due to the fact that grey wastewater is much less polluted than black wastewater and has a relatively high temperature of 30–35 °C. This allows for their use as an energy source. The energy recovered from wastewater is mainly used to preheat water. Suitable devices such as heat exchangers and heat pumps are used for this purpose.
Energy savings in swimming pool facilities can be achieved not only by using high-performance equipment but also by using equipment to recover heat from rinsing water, exhaust air, or renewable energy. In swimming pool facilities used year-round, it is difficult to talk about a complete replacement of conventional heat sources by renewable energy systems. Swimming pool water heating requires careful selection of the system to achieve the highest possible thermal comfort expected by the users and the purpose of the pool, with the lowest possible operating costs. For comfort of use and stable pool water temperature, an air-to-water heat pump can be an advantageous solution. Independently from the sun, it ensures any long working time and thus reaching the required pool water temperature. The solar system will ensure low operating costs.
Comparative calculation analyses can be carried out on the basis of comparative variants or adopted design assumptions. Calculation tools are available (e.g., Menerga Designer server), which enable the design of complete systems e.g., air conditioning for averaged weather conditions for temperature: −20, −15, −10, −5, 0, 5, 10, 15, 20, 25, 30 °C [107
In the case of the object analyzed in the manuscript, the investment return time is about 3 years or in the case of a sewage heat recovery system only with a recuperator, with assumed parameters about 5 years. Of course, other, perhaps more effective solutions can be proposed. Nevertheless, the authors focused on their chosen solution, which will be enriched in the near future with additional energy-saving schemes (an agreement to support investments from European Union funds was signed). These reflections will be the subject of future authors’ manuscripts.
The concept presented in the manuscript will be complemented by a solar kit for the energy purposes of pool water heating, technological water, shower water, and domestic hot water. For modernization, a system of flat-plate solar collectors will be used (Vitosol 200-F type SH2 collectors with min. 83%) (Viessmann, Allendorf, Germany) together with all additional equipment (e.g., circulation pump, regulator, expansion vessel, insulated pipes, collector fixing structure) with a 6 × 1000 L hot water tank [108
]. The priority of the system will be domestic hot water, the installed solar power is to be 422 kW. From the analysis of domestic hot water preparation in the solar system in the sports facility under analysis, it can be seen that solar collectors are a rational way of obtaining energy for domestic hot water preparation, but the condition for the profitability of the investment will be obtaining financial support from external sources.
In addition, due to the poor technical condition of the existing supply and exhaust ventilation systems, the units will be completely replaced with new units with high energy efficiency of heat recovery with recuperators at the level of min 85%. The current units have a declared heat recovery of 40% (cross-exchanger with metal membranes). For office and technical premises, a variant with central air conditioning and local Fan-Coil Units (FCU) (three ventilation systems) is planned [109
]. This made it possible to divide air-conditioned rooms by function and to share the operating costs of potential users.
The system serving such rooms as entrance hall, administration, cloakroom hall and hanging cloakroom, hall with staircase, main hall, corridor, rental room, and others is to be equipped with the supply and exhaust air handling unit with heat recovery on a highly efficient rotary heat exchanger VS-180-R-PHC (Ventus, Opole, Poland) with supply and exhaust air capacity Vn/w = 18,000/19,100 m3
/h. The system serving the water treatment room, a hypochlorite warehouse, is to be equipped with the supply air handling unit VS-30-R-H and extract air handling unit VS-30-R-V with supply and extract air capacity Vn/w = 3600 × 2F; 3200 m3
/h. The third system serving the men’s cloakroom, the women’s cloakroom, is to cooperate with the supply and exhaust air handling unit with heat recovery on the VS55-R-PH cross-exchanger with supply and exhaust air capacity Vn/w = 4850/5335 m3
/h. For the above-mentioned ventilation systems an air handling unit with the cooling capacity of 110 kW is to be installed—KOMPAKT ZR250x2 R407C/73AG unit [110
The air handling unit ensures proper temperatures and humidity in the pool hall and in the wet areas of the swimming pool in each period of the year. It is also important that the heat recovered from the air in the pool hall, including the heat of evaporation of the pool water, will be directed to the place where there is demand in each period of the year. This place is used to heat swimming pool water and drinking water.
The calculations included in the manuscript were made on the basis of information provided by the investor. The costs of the main elements of the analyzed system were also verified on the basis of documents available on the Internet on the implementation of energy investment audits. The investor did not agree to include in the manuscript detailed information on the plans of the sports facility, hydraulic schematics, and photographs taken on the facility.