Technology
Product Conception
When I was at the marina, my neighbor was preparing for a trip to sail all the way down the east coast. His “new wife” was perplexed when he informed her that they will not have hot water while sailing and at anchor. She informed him “they will have hot water”... So he purchased two 39 x 39 inch solar black pool heater mats from Home Depot with 200 btu output and attached them to each front corner of the catamaran. Not only did the once pretty catamaran take on a raccoon like appearance, but the most it did was raise water heater temperature from 70 to 85 F and was cold come morning.
Myself being a engineer, challenged myself to a better solution..
Navicular Solar vs. Current Technologies
My design requirements were simple, make a unit that is small enough to fit on a 27 ft sailboat behind the mast and produce up to 900 btu per hour reliably. This would enable a typical 4 to 6 gallon marine hot water tank to heat from 75 up to 140 F and would still have enough water temperature (rise vs heat loss) for morning showers and replenish hot water used the next day. Easier said than done...
There are only a few other options on the market for small solar hot water systems on boats. They use small conventional thermal solar flat-panels and one even connects a heat exchanger to the bottom of a PV panel. The latter although creative, is the least reliable with it’s environmentally exposed heat absorber and is not used in residential solar for that very reason. Standard thermal solar flat-panels have been the solar industry mainstay for many years but when applied to a limited boat size, the flat-panel performance drawbacks become critically magnified. Where flat-panel systems for boats fall short is in it’s environmental heat loss which limits water temperature to 110 F, performance degradation at various angles to the sun and cloudy day performance. The old residential solar flat-panel design adage, “just throw more collector area and bigger hot water tanks at it” doesn’t work well on a boat. There simply is not enough room.
Solar evacuated tubes are the latest in thermal solar technology and are known for their ability to produce hot water in cloudy conditions and guard against heat loss with a glass vacuum barrier. The following graph from wikipedia illustrates it perfectly.
When I was at the marina, my neighbor was preparing for a trip to sail all the way down the east coast. His “new wife” was perplexed when he informed her that they will not have hot water while sailing and at anchor. She informed him “they will have hot water”... So he purchased two 39 x 39 inch solar black pool heater mats from Home Depot with 200 btu output and attached them to each front corner of the catamaran. Not only did the once pretty catamaran take on a raccoon like appearance, but the most it did was raise water heater temperature from 70 to 85 F and was cold come morning.
Myself being a engineer, challenged myself to a better solution..
Navicular Solar vs. Current Technologies
My design requirements were simple, make a unit that is small enough to fit on a 27 ft sailboat behind the mast and produce up to 900 btu per hour reliably. This would enable a typical 4 to 6 gallon marine hot water tank to heat from 75 up to 140 F and would still have enough water temperature (rise vs heat loss) for morning showers and replenish hot water used the next day. Easier said than done...
There are only a few other options on the market for small solar hot water systems on boats. They use small conventional thermal solar flat-panels and one even connects a heat exchanger to the bottom of a PV panel. The latter although creative, is the least reliable with it’s environmentally exposed heat absorber and is not used in residential solar for that very reason. Standard thermal solar flat-panels have been the solar industry mainstay for many years but when applied to a limited boat size, the flat-panel performance drawbacks become critically magnified. Where flat-panel systems for boats fall short is in it’s environmental heat loss which limits water temperature to 110 F, performance degradation at various angles to the sun and cloudy day performance. The old residential solar flat-panel design adage, “just throw more collector area and bigger hot water tanks at it” doesn’t work well on a boat. There simply is not enough room.
Solar evacuated tubes are the latest in thermal solar technology and are known for their ability to produce hot water in cloudy conditions and guard against heat loss with a glass vacuum barrier. The following graph from wikipedia illustrates it perfectly.
As you can see in the graph, the flat-panel collector temperature drops with the solar radiation sensor intensity (cloudiness). The evacuated tube collector doesn’t drop much in temperature, just in loop pump frequency. Basically the flat-panel collector stop supplying heat to the system for about 4 hours. Evacuated tubes have a selective absorber coating and do not need direct sunlight to produce heat (although it produces much more heat in direct sunlight). In a home or residential flat-panel system, this periodic drop or cloud drop in flat-panels is ok because you can store heat in the geyser/storage tank which the graph shows. The slight loss in the graph labeled flat-panel geyser is simply the system storage heat loss without the addition of heat from the flat-panels. Solar hot water marine systems do not have the luxury of space like homes thus no “cloudy day storage.”
The Navicular technology uses a flat-panel and evacuated tube hybrid. Why use flat-panels if they perform so poorly in the previous graph? In some areas, flat-panels perform better than evacuated tubes. The next graph taken from the same wikipedia website above compares the two in more detail.
Solar flat-panels are very efficient in the 20 C Tm-Ta region. What this means is flat-panels produce a lot of heat from 75 to 110 F range if the ambient temperature was 75 F (even in some cloudy conditions). The reason this was not seen in the previous graph is because it was during heavy overcast while being around the 20 C Tm-Ta boundary. When you use the two technologies together you gain significant performance across the spectrum as a water heater heats from 70 to 140 F. The evacuated tubes produce much higher temperatures needed for limited marine tank size. Also another benefit of flat-panel with evacuated tubes is the flat-panel acts as a overheating brake and keeps water from boiling. In strictly evacuated tube systems, overheating is a problem and requires a cooling loop or controlled sizing/array placement. Solar flat-panels start to discharge excess heat beyond the “20 C Tm-Ta region” and keeps the Navicular system with evacuated tubes around 140 F max.
In addition to the hybrid panel-evacuated tube system, Navicular technology has both active electronic sun tracking and passive geometry sun tracking. Conventional flat-panels have no tracking and are pointed in a fixed position towards the sun. Evacuated tubes inherently have “passive sun tracking” due to their cylindrical shape as the sun arcs across the sky. The Navicular electronic active sun tracking controller has several pumps, senors and control valves enclosed in the collector. This allows different parts of the system to shut down and turn on as the sun arcs across the sky for continuous heating. The Navicular passive geometry tracking layout allows the majority of the evacuated tubes to receive sunlight in any angle (see side-view photo).
Side-View Photo
In addition to the hybrid panel-evacuated tube system, Navicular technology has both active electronic sun tracking and passive geometry sun tracking. Conventional flat-panels have no tracking and are pointed in a fixed position towards the sun. Evacuated tubes inherently have “passive sun tracking” due to their cylindrical shape as the sun arcs across the sky. The Navicular electronic active sun tracking controller has several pumps, senors and control valves enclosed in the collector. This allows different parts of the system to shut down and turn on as the sun arcs across the sky for continuous heating. The Navicular passive geometry tracking layout allows the majority of the evacuated tubes to receive sunlight in any angle (see side-view photo).
Side-View Photo
| navicular_system_calulator.xlsx |
Performance Data
I was surprised the first week I started testing this system when I had a day with hourly forecasts of 75% to 90% overcast all day. At first I didn’t think it would heat much if any with those overcast conditions. My SPA 15 Isotemp 4 gallon water heater started at 77 F and heated to 115 F by end of day with heavy overcast all day. *Normal operation on a given sunny summer day will bring this tank from 75 to 120 F within 2.5 hours and 140 F end of day.
Another key feature with the Navicular system is that it has an accelerated hot water recovery rate after use. The Navicular technology has a built in internal geyser with up to 1000 btu standby, which with the 900 btu hourly rate, can put up to 1900 btu back into the tank for 1 hour. This is equivalent to 75% of the 750 watt AC heating element in a standard Isotemp tank! A 4 gallon tank can recover quickly with hot water within an hour after first shower. In northern latitudes, you will be able to get up to 3 showers a day from the system and 4 showers a day in southern latitudes.
For performance profiles vs tank size please see attached spreadsheet under side-view photo.
*Be skeptical of (2) claims from other boat solar hot systems on the Internet. Per their diagram they are measuring a thermosyphon effect from the collector outlet tee to the faucet, not the center of the hot water tank. One claims to be able to bring water to 115+ F in a hour. At that btu per hour rate, the tank would be 250+ F end of day! If you buy that system I guess you can re-power your diesel with a steam engine as an option. Another from a Cruising Helmsman article claims the system to have water too hot to touch in 30 minutes which would be about 120 F. The Cruising Helmsman article is a good example of how not to connect a thermal solar system on a boat. There would be significant heat lost from thermal collector directly to the faucets.
I was surprised the first week I started testing this system when I had a day with hourly forecasts of 75% to 90% overcast all day. At first I didn’t think it would heat much if any with those overcast conditions. My SPA 15 Isotemp 4 gallon water heater started at 77 F and heated to 115 F by end of day with heavy overcast all day. *Normal operation on a given sunny summer day will bring this tank from 75 to 120 F within 2.5 hours and 140 F end of day.
Another key feature with the Navicular system is that it has an accelerated hot water recovery rate after use. The Navicular technology has a built in internal geyser with up to 1000 btu standby, which with the 900 btu hourly rate, can put up to 1900 btu back into the tank for 1 hour. This is equivalent to 75% of the 750 watt AC heating element in a standard Isotemp tank! A 4 gallon tank can recover quickly with hot water within an hour after first shower. In northern latitudes, you will be able to get up to 3 showers a day from the system and 4 showers a day in southern latitudes.
For performance profiles vs tank size please see attached spreadsheet under side-view photo.
*Be skeptical of (2) claims from other boat solar hot systems on the Internet. Per their diagram they are measuring a thermosyphon effect from the collector outlet tee to the faucet, not the center of the hot water tank. One claims to be able to bring water to 115+ F in a hour. At that btu per hour rate, the tank would be 250+ F end of day! If you buy that system I guess you can re-power your diesel with a steam engine as an option. Another from a Cruising Helmsman article claims the system to have water too hot to touch in 30 minutes which would be about 120 F. The Cruising Helmsman article is a good example of how not to connect a thermal solar system on a boat. There would be significant heat lost from thermal collector directly to the faucets.
