Hot Tub Heat Exchanger

Brazetek has shell and tube heat exchangers, suitable for hot tubs. The smallest is 55KBTUh. I will have to measure the compartment inside the hot tub to make sure it fits. I will also have to decide if I want to keep the hot tub electric heater or not.

[ Hot tub heat exchanger ]

Physical dimensions for the hydronic coil

The furnace area where I intent to insert the hydronic coil has a maximum inside dimension of 25"x25". The air channel is 22"x22".
The brazetek catalog has several dimensions for coils. There is a pdf that lists all dimensions. A 20"x20" coil will have 20"x20" coil area, and 24.5"x24.5" outside dimension, not counting the inlet/outlet pipes. This is the size that will fit. The inlet and outlet pipes will come out of the duct through pipes.
I looked for a coil that I could insert at an angle, to increase the coil area, but there is none that would fit, and it would make installation more difficult, so I will order a 20"x20" coil and install it in the furnace. The coil won't be plumbed yet, since there isn't a solar system yet to connect it to. There will be a 20"x20" paper filter installed against the coil, upstream of it. I may also install filters at the two inlet grilles. This will improve air quality and prevent the hydronic coil from clogging with dust, which would decrease its efficiency. Due to the low temperature of solar heated water, I will need the coil to keep optimal performances.

Solar Panel Set

The solar panel was set this afternoon. I was too busy and forgot to take pictures. The panel is very heavy. For the next panels, I will change the design to make it lighter, using 1x cedar instead of 2x pine for the frame, 1/2 inch plywood for the backing, or remove the plywood and replace it with narrow cedar planks under the pex pipes, the end of the aluminum heat spreaders will not be backed, which should be OK.

I still have to connect the panel to the hot tub.

Blue Moon

The Blue Moon, Friday morning. Magical light.

Sunsets on Mars, and on Earth

Curiosity lander shot its first sunset on Mars.

Compare this to a sunset on Earth:

One would have to make sure the zooming was the same, that the aspect ration was respected ... but it still gives a good appreciation about how the Sun would look like on Mars, compared to Earth.
In reality, Mars being on average 1.5 times further from the Sun than Earth, the Sun should appear 1.5 times smaller there.

First Solar Panel Started.

I spent the whole afternoon today building the solar collector to heat our hot tub. This is an experiment to familiarize myself with the technology. About half of the panel is built, and hopefully, I will have it running by the end of the weekend. First was putting the heat spreader plates on top of the plywood sheet and mark each pex run.

Each line is where the pex tubing will lay. This will help, particularly with the problem I will encounter in the next step.

At the end of the first pex run, I had to bend the pex tubing to lay on the next line. Just to be sure I would not kink the tube, I checked on line the minimum bending radius: 5". My mistake was to think diameter instead of radius. I have 5.5" between lines, so that is 2.75" radius, too short. The solution was to go every 2 or 3 lines, and come back on the next run. Here the first two lines are laid.

Next, the four fist lines laid.

Seven lines laid so far, three more to go.

One more to go. All this done with a bending radius no less than 5".

Now the painting, after cleaning the plates with alcohol. One spray can of black rustoleum is not going very far.

I went tonight and bought a liquid can of high temp black paint. Writing the blog, I realized that there will be a problem with this design. With the way I laid the lines, it won't drain properly. That is OK for a hot tub that can be drained for the winter. For the hot water heater however, I will have to get pex that can be bent with a 2.75" radius, or find another way, such as running two lines in parallel. And that is why I started my solar projects with an experiment :-)

Backup Heat

One option for backup heat is using a heat pump water heater, a commercial type that can handle the load. The problem with that option is that the heat pump is unlikely to be running at the time I will need it the most, because the outside air will be too low. However, thanks to the 1-day storage capacity, I can set the heat pump to run only during the warmest part of the day, one extended cycle to bring the storage tank temperature up to 140°F, once a day.
Unfortunately, all the water heater heat pumps I found online are designed to work indoor, with intake air temperature around 20C, or 70F. I haven't found the right product yet. The heat pump is an air-to-water heat pump, with intake air temperature as low as 35F.
Another option is to use a geothermal heat pump. The heat pump must be designed to heat water, not air. Most geothermal heat pumps heat air, but there are a few that heat water, such as the water furnace. Looking at their website, it appears they have a design that is suitable for my application: Model 084.
Water Furnace
According to the performance table in page 5, the entering source temperature may be as low s 32F, with an entering load temperature of 104F, giving a COP of 3.1. This COP probably does not include the energy used by the two water pumps (load ans source). Instead of being connected to ground loops, the heat pump will be connected to an air to water heat exchanger. There will be a gradient between the water and air temperature, that must be determined (it depends on size of exchanger, and output of heat pump).
The heat pump can be set to run only of the warmest part of the day, such as late afternoon. A simple way to do that is a timer. A more efficient way would include a smart controller retrieving the max temperature for the day from a weather forecast, and start the heat pump when the temperature is less than 5F below the max. The 5F gradient should be determined by the amount of time the heat pump will have to run to bring 270 gallons from 100F to 140F.
The month of December 2012 was colder than average December months in past years. If we assume a 5F gradient for the heat exchanger, minimum temperature requirement is 37F. Every day in December 2012 exceeded 37F at their max. That doesn't mean they exceeded for long enough to heat 270 gallons. We need to make another assumption for that, such as the max temperature of the day should be 5F higher than the min required temperature to provide enough heat (number TBD for now). Minimum outdoor requirement becomes 42F. 11 days were not suitable. For those days, the electric strips will have to be used. Assuming a COP of 3.1 for 20 days, the overall COP for the month is 3.1*20/31 = 2.

Storage tanks

The initial idea of a 1400 gallon storage tank in the main downstairs room appears today unpractical. The alternative is too have a tank in the water heater closet, and another one in the furnace room, which means two separate systems.
The water heater closet can accomodate a 3' x 3' x 4' tank, with a chamfer where the tank will come underneath the stairs. Total water volume is 75 gallons. Total energy storage, assuming a usable temperature range of 140F to 80F, is 10KWh. That is about 1 day of storage in winter.
The furnace closet can accomodate a 4' x 4' x 5' tank, with a water capacity of 270 gallons, and an energy storage (assuming 140F - 110F temp range) of 20KWh, equivalent to 1 day of heat in the winter.
The location of the furnace tank would make it possible to reclaim heat from the upstairs wood stove, which would be a significant advantage, since the wood stove easily overheats the upstairs living room.
Because the systems will be separated, they will also need separate solar panels. The furnace tank will use the garage South wall, with up to 150sqft collector area. The water heater system will use the deck rail, 75sqft collector area. An alternate location is the upstairs South wall, with also about 75sqft area.
Optimum solar fractions (not accounting for cloudy days) is 84% for hot water, and 72% for space heat. Total solar fraction (also accounting other uses than heat) is 39%. Yearly savings is $600. Actual savings will probably be between $400 and $500. With a total estimated cost at $2000, payback is between 4 and 5 years.
Our total yearly electric consumption will go from 25MWh to 15MWH a year. Further efforts on improved insulation (windows and doors are leaky, walls could be improved, siding needs resealing and repainting) may reduce that to 10MWh. Net zero would then be achieved with a 6KW photovoltaic system.