[Edited since yesterday, to move the main temp sensor to the top of the heat exchanger, to make the pump variable speed, and to vastly improve the drainback system]

Consider the following system:

On the roof, a conventional solar thermal collector ... My preference would be for an evacuated tube collector, to reduce the risk of it freezing in winter, but a flat plate collector would be fine in mild climates.

The collector has two pipes: an intake, and a return.

Inside the house, a heat exchanger between the solar heating fluid, and the domestic hot water.

At the top of the heat exchanger, where the gaseous heating fluid enters from the solar collector, a temperature sensor.

At the bottom of the heat exchanger, inside the heating fluid pipe, a sensor to detect the phase of the heating fluid (gas vs liquid).

After the sensors, a pump to send the fluid back up to the solar collector's intake, and a check valve, to prevent the water from running backwards through the pump, when the pump is turned off.

The pump is controlled by the liquid sensor and the temperature sensor: Whenever liquid is present, and its temperature is not above 190F, the pump will operate.

Furthermore, the pump's speed should be variable: when the temperature is 120F or below, it will operate at maximum speed. Between 120 and 190, the pump will operate at an intermediate speed, with hotter resulting in a slower pump speed, and cooler resulting in a faster pumping speed.

At the very highest point in the system, a check valve that can allow heating fluid out into the atmosphere, as a safeguard against the temperature sensor failing.

Fill the system with distilled water, then attach a vacuum pump to the check valve at the top of the system. Running this pump will first degas the system, then lower the pressure enough so that the water in the solar collector will boil. When a precisely calculated amount of water has been removed, the vacuum pump is removed from the system.

As a result, the solar fluid loop will contain only water and steam, and no other gasses. Because of how much water we removed from the system, it's pressure will be low, so it's boiling point will be low.

During the day, the heat of the sun will cause the water to boil. The resulting steam will flow up to the top of the collector, down the return pipe, into the top of the domestic water heat exchanger, and condense.

The condensate will activate the liquid sensor, activating the pump, which will move the liquid out of the heat exchanger and back to the solar collector.

At night, when the collector is colder than the heat exchanger, the water in the collector won't boil, of course. If there were any liquid distilled water in the heat exchanger, it *would* boil, but since that water is continuously removed to the collector by the pump, we can expect the heat exchanger to be "dry" when night falls.

Furthermore, because there's only enough liquid water in the system to fill the pipe from the pump's check valve, up to the top of the collector, the water in the collector will not overflow and run down to the heat exchanger. As a result, the liquid stays in the collector at night, and no heat is transferred to it from the domestic hot water heat exchanger.

With this design, the amount of pumping energy needed to one's drinking water is very small: Each pound of water pumped up to the solar collector is changed by the sun into a pound of steam. Each pound of steam condensed in the heat exchanger heats the domestic water by 1194 BTUs.

The only potential issue is freezing in winter, since the distilled water is left in the solar collector on the roof each night.

This can be solved by adding an automatic drainback system.

Firstly, at the bottom of the solar collector, there would be a temperature sensor. When the temp drops below 35F, the system would enter drainback mode, until manually reset.

Secondly, there'd be an insulated distilled water storage container. In normal operation (not in drainback mode), this tank would only have low pressure steam in it.

A pipe tee just above the main pump's check valve would lead to a solenoid valve that connects to the bottom of the storage tank. The top of the storage tank would be connected to another tee, in the pipe between the top of the heat exchanger and the top of the solar collector.

Whenever the solenoid valve is open, we have what's effectively a perfectly normal heat pipe, which moves heat from the inside of the storage tank, to the solar collector. Eventually, all of the liquid water ends up in the storage tank, with only steam in the solar collector and the heat exchanger.

As the system chills, the pressure of the steam and water will drop to water's triple point, and the temperature of the steam in the solar collector and the water in the drainback tank will drop to water's triple point. Because steam is a poor heat conductor, the water in the heat exchanger won't get chilled.

The steam remaining in the solar collector can potentially freeze (or rather undergo deposition), but because of how little steam there is (compared with how much liquid there would be without the drainback system) less damage can occur.

To leave drainback mode, we flip a switch.

This switch does two things: First, it closes the solenoid valve just above the pump's check valve.

Second, it opens a second solenoid valve, which is located below a tee between the first solenoid valve and the drainback tank. This second solenoid valve connects to a check valve, which leads to a pipe tee, which is located between the heat exchanger and the pump.

Assuming bottoms of the drainback tank and the heat exchanger are at approximately the same heights, or if the drainback tank is a bit above the heat exchanger, gravity will move the water from the tank to the bottom of the heat exchanger, where it will trigger the liquid sensor, which will turn on the pump, and move the water into the solar collector.
-- goldbb, Sep 29 2009

Nicely thought out, but as I understand the idea you've got low pressure and hence low temperature steam transferring heat into the domestic hot water system.

What heat?
-- egbert, Oct 02 2009


The pressure is below atmospheric pressure, but that only means that it's boiling point is below 212F, not that it's cold.

The heat is transferred from collector to heat exchanger by means of the latent heat of vaporization of the steam.

Imagine it's night, shortly before sunrise.

The solar collector is full of cool water, the heat exchanger is full of steam. The pressure of the water and steam is very low, and the domestic water in the heat exchanger is much warmer than the boiling point of the steam. Nothing much is happening, and the system is at equilibrium. Even if domestic water is in use, and is flowing through the heat exchanger, not much happens, because steam is a thermal insulator... the little bit of steam in the heat exchanger might absorb a bit of heat from the drinking water, but it won't go anywhere with it (it won't conduct it away, since steam is a poor insulator, and it can't convect it away, since the pipe from the top of the heat exchanger to the top of the solar collector is too thin for a convection cell).

Now imagine that the sun comes up.

The sun warms the collector, and since the pressure inside is very low, the water begins to boil.

As the water in the solar collector boils, it raises the pressure (and the temperature) of the steam. The steam pressure will continue rising, until it's boiling point is above the temperature of the domestic water in the heat exchanger.

When this happens, since the domestic water's temperature is below the temperature of the steam, and because the steam's temperature is always exactly at it's boiling point, the steam in the heat exchanger will condense, releasing heat, and transferring that heat into the domestic hot water.

The rate of condensation in the heat exchanger and boiling in the solar collector will reach a balance, preventing the steam pressure from becoming excessive.

In particular, as long as the temperature of the domestic hot water doesn't get above 212F, the pressure of the steam won't get above atmospheric pressure. As a result, the thermostat switch for the condensate pump prevents the steam's pressure from rising above atmospheric pressure.

Similarly, as long as the pressure of the steam doesn't get above atmospheric pressure, the temperature of the domestic water can't get above 212F. The check valve at the top of the system ensures that if (due to a failure of the thermostat switch) the steam pressure rises *to* atmospheric pressure, steam will be released (through the check valve) to the atmosphere, and pressure will not rise further. Thus, the check valve ensures that the domestic hot water never gets above 212F.
-- goldbb, Oct 04 2009



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