Evacuated Tube Hot Water System
My new hot water system is comprised of 30 roof mounted glass tubes. Each tube has a copper liquid filled heating element inside the vacuum tube. These tubes are capable of absorbing up to 92% of the heat from the sun. They absorb up to 40% more heat than a flat plate collector of the same surface area.
The system is also fitted with a 3600W element to provide back up during prolonged cloudy or cold weather or unusually high water usage. The back up heating element can be connected to your off peak power. I am hopeful to leave the back up off all year or at least for 8-10 months a year.
The heat from the enclosed copper pipes rises and is transferred into the circulating water in the manifold which is located at the top of the tubes. The heated water in the manifold is then periodically circulated down to the SS storage tank at ground level by a 30W electric circulating pump.
A 2W controller monitors the manifold and storage tank temperatures and periodically turns the pump on & off. In all, the pump is on a couple hours per day. The pump & controller must be connected to your normal power supply. The controller operates in three modes:
1) Freeze protection mode: The pump comes on to circulate warm water if the manifold water temperature drops to 3C and turns off when it reaches 5C.
2) Hot water circulation mode: The hot water produced in the manifold is circulated down to the storage tank when its water temperature is 9C hotter than the tank temperature. It shuts off when the temperature difference is 5C or less.
3) High storage tank temperature protection mode. The pump will remain off once the tank temperature reaches 74C.
There is a pressure temperature relief valve on both the manifold and storage tank to vent steam should the temperature in either location reach 99C.
There is also a solar tempering valve which mixes cold water with the tank water to maintain the water temperature to your taps at 50C to prevent the danger of scalding.
Prior to installing the solar hot water system, I was using an average of 3.3 kWh during the summer and 4.4 kWh in winter, roughly 4.0 kWh per day or 1460 kWh a year. At the current price of off peak power this amounts to $91/ year. This is for a household of one adult. I would think the cost would increase fairly proportionally for larger households (however there would be heat losses through the tank as well).
After monitoring the power consumption for the month of March I can report that it used 4.45 kWh/ month (148Wh/day) to power the pump & controller. The average daily power rose from 148 to 190Wh/day while there was two adults in the house. On cloudy days, it used less power (since the temperature difference between the manifold & the storage tank exceeded 9C on less occasions).
After a cloudy spell, it would use above average power to bring back the storage tank water to maximum temperature. Overall, it ran most efficiently on successive clear sunny days.
The cost to run the controller & pump is about $9/ year so my net saving on my power bill will be about $82/ year. No doubt this saving would be 2-3 times greater for an average family usage pattern.
Clearly the cost of running the controller & pump off grid power is very minimal. We are sometimes asked if this type of hot water system is recommended for someone on a solar PV stand alone power system. We'd suggest you would normally be better off using a flat plate collector gravity fed type of hot water system if you are on a stand alone PV power system as you would probably need at least an 87W solar panel to meet the power load from an evacuated tube hot water system. Your inverter would need to be on run mode rather than the power saving standby mode.
I will mention that Barbara from New Zealand made an 'experimental' model running a DC pump direct off a 30W solar panel. I decided not to reproduce her email here as any Aussie would need to use a commercial model to get the generous government rebates for a hot water system. However, if you are interested to read it, let me know & I will forward the text of her email to you.
Lastly, I should mention that my water has always been 'steaming' hot even after a few days of mostly cloudy weather. I will update you mid winter as to its performance and power usage during that time.
This article was first published in issue #124 of our E-Newsletter and updated since.