- right now, the project has been canceled due to regulations and liability issues with placing a body of water on state property (Humboldt State University).
Introduction
The CCAT solar heated hot tub will take advantage of the properties of water density. Anyone who slowly wades into a small body of water on a hot, sunny day will notice the temperature change as they go from the surface to deeper down, with hot water near the surface and cold water below it. This is due to the convective movement of water, and in our case, eliminates the need for a water pump. Using this property, I plan to design and build a solar heated hot tub at CCAT using at least one of their solar water heating panels as well as the materials listed under budget. It is expected that this hot tub will use an indirect heating system, meaning the water being heated by the sun will go on to heat the water in the tub. This is so that the human scuzz doesn’t find it’s way into the tubing and reduce efficiency. A filter may be incorporated to solve this problem of sanitation. The tub should be large enough to comfortably fit four people. Some sort of thermal cover with a latching device will also be used to help heat during the day, retain heat during the night, and for security purposes.
Criteria
- This hot tub will have to hold up it's name and be hot. A temperature of at least 90 degrees Fahrenheit is a main goal in this project.
- Since the project is at CCAT, another primary goal is to come up with a presentable design.
- A primary goal is to find some sort of thermal cover to help heat during the day and also to retain heat during the night. Regulations state that the tub must have a latching top so this cover must also be equipped with a locking device.
- The cost to construct this project should remain below $700. This price is the lowest I could find of a regular, gas-powered water heater-heated hot tub. This is another main goal.
Budget
| Quantity | Product | Location | Total Cost |
|---|---|---|---|
| 2 | Solar water heating collectors (1 large, 1 small) | CCAT | $0 |
| 1 | 3/4" Watts L70A-T Lo-Temp Threaded Tempering Valve (100°-130°) | pexsupply.com | $76.35 |
| 1 | 3/4" Red HydroPEX Tubing (100 ft. coil) | pexsupply.com | $42.95 |
| 1 | Box of 5 Viega 3/4" Propress Unions | pexsupply.com | $64.85 |
| 2 | 3/4" Threaded Swing Check Valve | pexsupply.com | $13.98 |
| ~20 | 2x6 planks | HSU construction site | $0??? |
| 2-3 | Ring clamps | rhtubs.com | $0??? |
Design
Tentative Timeline
- Week o' March 8 - Research
- Week o' March 15 - Research. Ask CCAT questions regarding their panels/location/water heater. Budget and literature review completion and submission. Develop rough draft of design. Email design to CCAT for feedback. Possibly make a trip to the hardware store for some materials.
- Week o' March 22 - Research. Talk to CCAT about their opinions/ideas and possibly choose hot tub site. Materials being obtained/ordered.
- Week o' March 29 - Research. Finalize design. Start leveling site for tub.
- Week o' April 5 -
- Week o' April 12 -
- Week o' April 19 -
- Week o' April 26 -
Literature Review
Solar water heating is an old practice. Using some of today's technology, we are able to create extremely efficient systems depending on the climate of a specific region.
Water
- Water weighs about 8.4 pounds per gallon.
- Water is extremely good at retaining and transferring its heat, making it one of the best solar fluids.
- When water is heated, it because less dense and will rise above colder water. (See passive systems)
- Water is densest just before it freezes and then becomes less dense as it gets colder. This is why ice floats.
- When water falls below its freezing temperature, it expands and has the potential to break things like rocks and plumbing.
- Most water in the U.S. that comes from the tap is "hard," meaning it has dissolved minerals in it. Heating hard water solidifies these minerals. This is a bad thing in solar water heating systems because the solidified minerals create resistance in the plumbing, storage tanks, etc.
Types of Systems
- Flooded systems have water in them at all times.
- Systems where the hot water being used was heated in a solar collector are classified as direct systems. In other words, the hot water being used is heated directly by the sun.
- In indirect systems, water or any other heat transfer fluid is heated in a solar collector. This hot solar fluid then goes on to a heat exchanger, which heats the water being used for domestic purposes.
- An open system is a system where "new" water is circulated in the system.
- A closed system is simply the opposite of an open system; the system does not gain or receive outside water.
- Passive systems are systems that do not use external energy to circulate the water.
- Alternatively, systems that use external energy for pumps and controls are referred to as active systems.
Solar Collectors
Different solar collectors are used depending on the system's purpose and location. It is important that the materials used in these heat collecting devices can withstand high temperatures. This means plastics aren't usually a good idea in collectors.
- ICS units - Integral Collector Storage (ICS) units are basically a combination of a solar collector and a storage tank. These steel or copper tanks sit in an insulated box that acts like a greenhouse when absorbing the suns heat. Black nickel has proven to be one of the best absorber coatings for these collectors and the ones listed below. ICS units come in a single tank variety where there is only one storage tank as well as a tube variety where there are multiple storage tanks in the absorber box - these tubes have more surface area exposed to the sun so they heat up and cool down quicker than the single tank-type. These units are common in places that do not have freezing temperatures.
- Flat plate collectors - these are the most common of the solar absorbers. They feature a housing similar to the ICS units. Cold water flows into one end of the collector and is then dispersed into multiple copper tubes where it heats up. The hot water then collects in a single tube and comes out of the other end. These are a smart choice in climates that have snow because the snow slides off easily.
- Evacuated tube collectors - these have a similar set up to flat plate collectors with a series of tubes. Their tubes are made of glass and inside each is an absorber plate. When manufactured, a vacuum is created in these tubes for the purpose of insulation. Because of this great insulation, the water in these collectors can reach dangerously hot temperatures.
Piping
Various grades and thicknesses of copper piping are standard for hot water systems because they can withstand high temperatures and are easy to install. At the hottest locations in the system, copper piping seems to be the most durable material available. Plastic alternatives like PEX will work in systems, but aren't as resistant to weathering, heat damage, etc. Pipe insulation also needs to be durable enough to withstand and keep in high temperatures. For outdoor applications, rubber insulators are a good choice, but to get the most out of them and make them last the longest, they need a jacket like PVC or aluminum jacketing.
Other Parts
- Storage tanks - these well-insulated tanks come in handy when hot water is needed during the dark parts of the day. Insulation most often would be a foam jacket on the outside of the tank and glass lining on the inside. Some tanks have steel bodies and newer ones have fiberglass bodies. Fiberglass tanks will not corode or rust like in the case of steel tanks. Steel tanks can also come with built-in heat exchangers. While this is convenient, they are not as efficient as external heat exchangers. Tanks without heat exchangers are more commonly used.
- Heat exchangers - these come in a number of designs, but basically all run water and the solar fluid in their separate pipes past each other in opposite directions to heat up the water.
- Expansion tanks - when water heats up and expands in a hard metal pipe, something has gotta give. This is where expansion tanks can help with the use of air inside a bladder inside the tank. When pressure builds, the water is able to fill the tank and compress the air.
- Pressure relief valves - these are spring-operated valves. When enough pressure builds in a system and the spring compresses, this opens the valve and releases pressure. It is important for the pressure release opening to be pointed away from where vulnerable objects might be, as hot steam will be blowing out of this opening.
- Unions - these are the pieces that join piping to other system components. These quick-release unions allow the system to be taken apart without having to cut the pipes.
- Check valves - these allow water to flow in only one direction through them. They do this with a swinging metal cap that swings shut when water starts to flow in the wrong direction. These swinging door type check valves will not work if placed where the flow is flowing down at an angle. Another type of check valve is a spring type where the flow of water compresses a spring to create an open flow. The spring decompresses and cuts off the flow when water goes in the wrong direction. This type of check valve will work in any configuration, but their springs are normally rigid and create resistance in the system.
- Tempering valve - these mix hot and cold water to end up with a specific ideal temperature, which is set using a dial on the valve.
Solar Heated Hot Tubs
Solar hot tubs are commonly created from the need for a dump load of an already existing solar water heating system.
Works Consulted
- Nusz, Benjamin, and Bob Ramlow. Solar Water Heating: A Comprehensive Guide to Solar Water And Space Heating Systems (Mother Earth News Wiser Living Series). Gabriola Island, BC: New Society Publishers, 2006.
- parameters. "Rubatex.com - Rubatex International, LLC - HOME." Rubatex.com - Rubatex International, LLC. http://www.rubatex.com/index.php?option=com_frontpage&Itemid=1 (accessed March 20, 2009).
Conclusions
After talking with Plant Ops, the project has been canceled due to regulations and liability issues with placing a body of water on state property (Humboldt State University).