7. Solar swimming pool heater system

Transcription

1 7. Solar swimming pool heater system 7.1 Introduction: Pool heating is a very good solar application. The systems are simple and relatively inexpensive. Pool systems usually use simple, low cost, unglazed plastic collectors. The pool itself is the thermal storage for the system, and the pump is used for filtering pool water will also circulate water through the solar collectors. Unglazed collectors (solar panels) which are the simplest and least expensive of all collectors are used almost universally for heating outdoor pools and space. The heating system itself is normally already required for pools, regardless of whether a solar heating system is added. Most of the heat loss from a pool is directly from the surface of the water through direct conduction and convection to the air, and through evaporation. If the temperature of the water is much higher than the temperature of the surrounding air (called the "ambient" temperature) surface losses will become very large. Thus, it is not practical by any means of heating to maintain the pool temperature more than 15 or 20 degrees above ambient temperature. Even if it were practical, few persons would want to swim in 70 degree water when the air temperature is 40 degrees. If the air temperature is comfortable, the water temperature cannot be comfortable at a much higher temperature. For these reasons, it is not generally desirable to heat a pool more than a few degrees below or above ambient air temperature. In IIT Roorkee, the swimming pool is open only for a short duration due to the vagaries of the weather. To achieve the goal of having an all weather swimming pool in order to ensure that everyone may benefit from it, a pool heating solution is being considered, the details of which are presented herewith. Solarification of IIT Roorkee Campus Page 94

2 7.2 Site Description: Fig 7.1: Photos of Installed Solar Pool Heating Systems The dimensions of the pool are: Length: 50m, Width: 15m, Height: 1.5m to 6m (Linear variation) The solar pool water heaters require a large amount of area for their installation; the pool has a good amount of area surrounding it (approx 800 sqm) and an adjoined spare area of approximately 300 sqm. The plan of the swimming pool is attached as S-1. Of this available area, we can use around 750 sqm area for installing the heaters. The area is flat ground with ample amount of sunshine available for the heaters. Solarification of IIT Roorkee Campus Page 95

3 7.3 Technology Description: Fig 7.2: Basic elements of a solar pool heater Pool water is pumped through the filter and then through the solar collector(s), where it is heated before it is returned to the pool. In hot climates, the collector(s) can also be used to cool the pool during peak summer months by circulating the water through the collector(s) at night. Some systems include sensors and an automatic or manual valve to divert water through the collector(s) when the collector temperature is sufficiently greater than the pool temperature. When the collector temperature is similar to the pool temperature, filtered water simply bypasses the collector(s) and is returned to the pool. Solar pool collectors are made out of different materials. The type one needs depends on the climate and how one intends to use the collector. If one will only be using the pool when temperatures are above freezing, then probably only need an unglazed collector system. Unglazed collectors don't include a glass covering (glazing). They are generally made of heavyduty rubber or plastic treated with an ultraviolet (UV) light inhibitor to extend the life of the panels. Because of their inexpensive parts and simple design, unglazed collectors are usually less expensive than glazed collectors. These unglazed systems can even work for indoor pools in cold climates if the system is designed to drain back to the pool when not in use. Even if you have to shut the system down during cold weather, unglazed collectors may be more cost effective than installing a more expensive glazed collector system. Solarification of IIT Roorkee Campus Page 96

4 Fig. 7.3: Example of how a solar collector works. Glazed collector systems are generally made of copper tubing on an aluminium plate with an iron-tempered glass covering, which increases their cost. In colder weather, glazed collector systems with heat exchangers and transfer fluids capture solar heat more efficiently than unglazed systems. Therefore, they can be used year-round in many climates. Glazed collectors also can be used to heat domestic hot water year-round. 7.4 Components of the proposed system: The heating system consists of the following basic subsystems: A. Control subsystem B. Optional backup heater C. Check Valve D. Pool Cover E. Filter F. Pump G. Circulation subsystem H. Collector subsystem A. Control subsystem: The control subsystem consists of a differential thermostat control for the circulation subsystem. A temperature sensor on a roof detects the temperature available at the collector and transmits this information to the control unit to be compared with the temperature of the pool water. When the collector temperature is higher than the temperature of the water by a predetermined amount, the control unit turns on the collector pump or actuates the control valve(s). This causes the water to circulate through the warm collector and to be delivered to the pool. If the temperature of the collector falls below the required difference, the control unit causes the collectors to be bypassed to prevent the water from being cooled. If a heat exchanger is not used, the collectors automatically drain back into the pool (from elevated units with positive drainage) when circulation to them is interrupted. B. Optional backup heater: For the most economical heating system with high reliability, a fired backup heater should be used. The backup heater automatically maintains the pool temperature during periods of low insulation, and also provides quick recovery capability. The Solarification of IIT Roorkee Campus Page 97

5 backup heater also improves the average efficiency of the solar system, since the solar system will provide pre-heated water to the fired heater, which would not otherwise be usable in periods of low insulation. C. Check Valve: The check valve used to prevent direct drain down of the collectors back through the filter D. Pool cover: Using a good pool cover is the most effective thing that can be done to save pool heating costs and reduce greenhouse gas emissions. Fig.7.4: Outdoor pool energy loss characteristics E. Filter & Pump: Solar pool heaters are integrated into the normal pool pump/filter system and work by intercepting the water flow after it comes out of the regular pool filter, and directing it through the solar collectors before it is returned to the swimming pool. The system uses the standard pool pump and filter. F. Sensor: To control the temperature of the pool water, a temperature sensor can determine whether or not additional heat is needed. When no heat is needed, a diverting valve is activated and the solar collectors are avoided - in that case the water flows directly back to the pool after filtration. 7.5 Methodology: For solar swimming pool heating unglazed collectors are preferred because during a realistic outdoor swimming season, while solar collectors are operating, the air temperature is frequently higher than the pool temperature, so the efficiency becomes higher for unglazed collectors than for glazed collectors. Since the temperature difference will always be maintained at about 20 degrees or less, the efficiency of unglazed collectors will always be near that of the glazed collector, or higher. Solarification of IIT Roorkee Campus Page 98

6 Conduction and convection losses are greater for the unglazed collector, but they are smaller than the transmission losses caused by the glazing on the glazed collector, unless there is a high wind to drive forced convection. There are a number of factors to consider in sizing the collector area and the first is the actual pool surface area. Starting with a minimum area equal to 100% of the pool s square footage; the system design is further affected by collector orientation, shading of the pool, local climate and length of swimming season desired. Ideally the collectors should face south and be tilted to latitude minus 10 to 15 degrees. Increase collector area to 75% if collectors are laid flat. Increase to 75% if collectors face west. Other orientations are not recommended. Although roof installations are most common, a frame can be constructed for "ground mount" for most solar systems when a roof installation is problematic due to inadequate space or aesthetic considerations. In areas subject to winter freezing, the collectors and plumbing should be installed to allow all water to drain when the system is off. The most practical and economic step in extending the swimming season is the use of a pool cover. As much as 70% of all the heat gained by a pool is lost through evaporation and when you include losses from radiation and convection the idea of heating a pool without a cover can be compared to heating a home with all the doors and windows open. Although applying and removing a cover can be inconvenient, the cover not only helps extend the swimming season it also keeps the pool cleaner and reduces chemical use. A transparent cover also allows passive solar heat gain during the day. Prices for a plastic composite cover range from to rupees per square foot and last from 2 to 5 years. The initial heating time shall be 4-6 days using the solar panel only to get the desired temperature of about 26o C. 7.6 Results Solar pool heating panels Dimension of pool Length: 50m, Width: 15m, Height: 1.5m to 6m (Linear variation) Collector area required is 100% of the area of the surface of the pool, as the collectors are to be placed on a flat surface. Tilt angle for each collector is taken to be 15 o to the ground, and panel size used is 4*12 feet. The effective dimensions of the panel after taking the tilt into consideration are 3.5*1.25 meters. Thus, 165 collectors are required for the purpose. According to the area availability we are able to place 170 collectors near the pool. Solarification of IIT Roorkee Campus Page 99

7 Pool surface area(m 2 ) Collector area required(m 2 ) Area of one collector(m 2 ) Number of collectors required Number of collectors placed Savings: The time duration for which pool heating is required is considered to be six months (Oct-March) since the rest of the time, summer/monsoon season prevails. Estimates of the savings from the solar pool heating are shown in the table: Equivalent electric heater required Operation cost(per kwh) Total operation( hrs) in a year System cost Cost saved in a year Energy saved in a year (6 hrs/day) 41 KW Rs Rs 37,40,000 Rs 1,36, KWh Thus, units of electricity can be saved annually by the installation of the solar water heating system for pool heating. 7.7 Conclusion This means that while the system s savings may not be good enough to guarantee a quick payback period, the system has still achieved its purpose. The ultimate aim of installing this system is to make the swimming pool accessible throughout the year and this will be achieved at a low capital expense. Thus 100% of the pool heating can be achieved without the need of the auxiliary heater if the panels are placed as shown on the plan. Without the need of much additional infrastructure, the dream of having an all weather swimming pool can be achieved. Solarification of IIT Roorkee Campus Page 100