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Solar hot water tubes – A cool item for hot showers

by Barb Peck & Bjarne Hansen 13 Mar 2019 15:27 UTC
Solar hot water tubes – A cool item for hot showers © Barb Peck & Bjarne Hansen

Brrrrrr! Oooooh, that water is COLD! But the crew thinks I need a shower, or they will mutiny...

Have you ever faced this dilemma? You might try a black solar shower bag – after all, they carry the warning: "Portable Shower can attain temperatures over 120 degreesF (50 C)". Sounds promising, but in reality they take a long time to heat up, even in full sun, and a breeze blows away any hope of reaching temperatures much over ambient.

We've improved upon the ordinary solar shower, and have been enjoying hot showers for the past 3 years. They are still dispensed from the solar shower bag, but some water is preheated and added to the rest of the bag's contents just before the shower. "Great idea, Einstein," you say, "lots of folks have realized that boiling water from a kettle is a convenient way of supplementing the shower bag's warmth." What about sticking with the solar theme and not burning any fossils?

Perhaps you have seen solar hot water tubes arrayed on roofs. It turns out these same high-tech solar tubes can be purchased in shorter lengths and are great for hikers, campers, and even boaters who want really hot water from the sun. Available sizes range from 250 ml (approx 1 cup) to 1 litre (4 cups). We bought three 700 ml tubes that are 51 cm (20 inches) long. We gave one to a friend and brought the other two with us on Hoku Pa'a.

Since the solar tubes are made of glass, we constructed a triangular wooden box, reminiscent of a Toblerone bar, for each tube to protect them when the sailing gets bouncy. The boxes are varnished 3/16-inch plywood with brass hinges so they close up for stowage and deploy with the two side 'wings' unfurled. Each tube is cushioned by two pieces of foam (salvaged from a child's play mat) held in place with 3M4200 adhesive. Several squares of scrap neoprene is attached to the two wings of the box to provide additional cushioning when it is closed. As a bonus, we used contact cement to adhere aluminum foil to the inside of the wings of the box to reflect even more sun onto the tube and speed heating.

How well do they work? We can boil water in 60 minutes, when the tubes are exposed to the midday sun in Mexico during the winter. When it's cloudy, it takes longer. However, if you can cast a shadow, you can obtain water that's too hot to touch.

These solar tubes are amazingly efficient. They are double-walled glass vacuum tubes, coated for very high absorbance on the outside and heat reflection on the inside. Cold air, or wind, does not affect the efficiency. Their insulation is so good that the tube does not feel warm to the touch even when the water inside is boiling.

On days when we want a shower, we fill each tube with water and secure it on deck facing the sun. If it's windy, we tie the tube and the wings down so they don't blow closed. We then go snorkeling, return to the boat, pour the hot water into our shower bag, and mix it with a few litres of cold water to obtain a comfortable temperature.

The solar tubes are constructed of glass, so other liquids can be heated up inside of them, even food can be cooked in them. However, not wanting to have to clean the inside of the tubes, we use them to boil plain water. This works great for tea, doing dishes, or filling a hot-water bottle when sleeping in colder climes.

The solar tubes are available on Amazon and eBay, and probably other online sources. Search for, "solar hot water tubes", or similar key words. We paid $45 for our set of three (in 2015),and $15 each still seems to be about the current cost. Some vendors include storage cases, which may be useful, but you will still require a method to hold them in place while in use. Most tubes come with a silicone stopper or a lid. You should puncture a small hole in the lid (if it doesn't already have one), otherwise pressure from the boiling water will pop the lid off.

An additional handy item, which we haven't seen for sale, is a pouring spout. It's sometimes hard to aim the stream of boiling water into the shower bag without spilling (or scalding!). We fabricated a set of what we call, 'lips', from two-part silicone molding putty (available in hobby stores). After kneading the two parts together, we formed it into a spout to fit around the top of the tube and let it set for a few hours. Now we push these lips onto the tube whenever we need to pour.

We hope this article has inspired you to have a comfortable warm shower using water heated by the sun!

This article has been provided by the courtesy of Bluewater Cruising Association.

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