Saturday, December 12, 2009

Solar Hot Water Part I - Fundamentals

So this blog begins a series of posts about solar hot water systems. Solar hot water is one of my favorite alternative energy systems because it is (theoretically at least) relatively inexpensive and it seems quite natural - put water out in the hot sun and let it heat up. Yet, very few people have installed solar hot water, even though such systems were quite common in California prior to WWII. In this series, I will go through the basics of solar hot water systems and describe the system we had installed this summer. I'll also talk about financials and some of the pitfalls involved in our installation experience.

This first post is about the basics of solar hot water systems.

In principle, nothing could be simpler. Unlike solar PV systems for generating electricity, solar hot water systems don't depend on any fancy quantum physics, they just depend on good plumbing. The idea is to put something on your roof (or somewhere else that gets a lot of sun) which traps heat from the sun, converting light into heat by absorption on a black surface, then transfers that heat to a fluid. That fluid forms hot water for domestic use. Solar hot water heating can also be used for space heating in winter, by running the hot water through hydronic heating loop pipes embedded in the floor.

There are two basic kinds of solar hot water systems: direct and indirect. In direct systems, the fluid heated in the thing on the roof, the collector, is the actual water that you will use for your domestic hot water. In indirect systems, the fluid is typically propylene glycol, a nontoxic fluid that is the basic component of antifreeze in automobiles (antifreeze also has toxic components too for rust inhibition and other purposes but those are not in the fluid used for solar hot water systems). Propylene glycol is used because the range of temperatures over which it remains fluid is much higher than for water. For example, a 60%  propylene  glycol/water solution freezes at -55F rather than 32F. Also, the same solution of propylene glycol/water doesn't boil until around 225F instead of 212F. This means that the pipes and collector are protected both  against freezing temperatures at night in winter and boiling hot summer days when your hot water tank already has enough heat and the system shuts itself off. The water or propylene glycol  in the pipe loop between the collector and your hot water tank is called the heat transfer fluid because it transfers the heat from the hot collector to the water in the tank (either directly if water or indirectly - through a heat exchanger coil in the tank - if propylene glycol).

The cheapest way to obtain a direct solar hot water system is to build it and install it yourself. Take a used gas or electric hot water heater tank, paint it black, put it into a box with a glass top, and plumb it into the domestic hot water loop in your house. Another variation is a do-it-yourself collector made out of a glass box and copper tubing painted black with the hot water from the collector running down from the roof through a box set in the ground filled with rocks.  The rocks act to collect heat while the sun is shining, and radiate heat into the water loop when it's not. There are many sites on the Internet where you can find plans for systems you can build for under $1000. These systems are not very efficient but they certainly are cheap. I'll not recommend any here, because I've not tried them, but if you are handy with tools, don't have a lot of money, and don't particularly care about performance, you might want to give it a try.

If you have more money to spend, there are three kinds of commercially available direct systems - thermosyphon systems, integrated collector storage systems, and drain back or drain down systems. Thermosyphon systems have no active pumping or other means of forcing the water to circulate between the collector and the hot water tank. Instead, thermosyphon systems depend on the tendency of hot water to rise off the collector. They have a tank attached to the collector usually around 40 gallons where the hot water collects. You can see the tank at the top of the collector in the picture below:

Cold water enters the collector at the bottom and rises up as it is heated. The hot water is collected in the tank at the top of the collector and from there goes  to the domestic hot water system, usually into the backup heater tank.

Integrated collector storage (ICS) systems are similar to thermosyphon systems in that they have some water storage in the pipes making up the collector. Unlike thermosyphon systems, however, the water pressure from the city main or well drives hot water into an auxiliary storage tank in the house.

Drain back or drain down systems also use the main water pressure to push water through the collector, but they use a different sort of freeze protection. Thermosyphon and ICS systems have a temperature sensor on the collector. When the temperature reaches the freezing point, the sensor causes hot water to be circulated into the collector to prevent freezing. This decreased the efficiency because in winter the hot water may come from the backup electric or gas hot water heater. In drain back or drain down systems,  when the temperature nears the freezing point a valve on the main side  of the collector closes. A valve on the house side closes too and a valve emptying the system opens. All the water in the collector and collector loop plumbing drains out. When the temperature rises sufficiently above freezing, the drain valve closes and the main and house valves open and the loop repressurizes.

The most expensive type of system is the indirect system. Indirect systems don't need to have freeze protection since the heat transfer fluid won't freeze in winter. An electrically driven pump drives the heat transfer fluid between the hot collector and a heat exchanger coil in a water storage tank. The heat from the heat transfer fluid is absorbed by the water in the tank, and the water then is used for the domestic hot water supply. When the temperature of the collector drops below the temperature of the tank, the pump shuts off. Similarly, if the temperature in the tank reaches a maximum (usually no more than 180F) the pump shuts off and the collector stagnates. At stagnation, the collector can reach very high temperatures, usually near the boiling point of water. Indirect systems need to have some kind of protection against stagnation, otherwise the heat transfer fluid can cook and become acidic. If that happens, the fluid must be replaced.

There are two kinds of collectors available for indirect and drainback systems: flat plate and evacuated tube. Flat plate collectors look like big flat glass skylights, except they are black. Here's a picture:

The simplest flat plate collector is just a piece of glass on top of a metal manifold or metal tubing coil through which the heat transfer fluid circulates. The manifold or tubing is painted black. Usually, insulation separates the manifold from the back of the collector to prevent heat from being transferred onto the roof.

 An evacuated tube collector consists of a collection of glass tubes from which all the air has been removed. Here's a picture:

The collector tubes extend vertically along the roof between a manifold on the top of the collector through which the heat transfer fluid circulates. A black heat pipe extends through the middle of the tube and contains another heat transfer fluid sealed in the heat pipe. When the sunlight strikes the collector, it heats up the heat pipe causing the fluid to rise to the manifold. Heat doesn't escape from the collector because there is no air inside to transfer heat, just as in a vacuum bottle. The heat exchanger fluid running through the manifold absorbs the heat from the heat pipe and transfers it into the hot water tank in the house.

Evacuated tube collectors put out more heat during the winter than flat plate collectors, and consequently are better for hydronic space heating systems, which circulate water through pipes in the floor. On the other hand, evacuated tube collectors tend to become too hot in the summer. Ironically, one of the major problems with solar heating systems is that they tend to get too hot in the summer. For domestic hot water systems, this isn't such a problem because you can size the system for maximum summer temperatures and limit the number of times the system stagnates. For solar space heating systems, however, you need somewhere to dump the heat, like a hot tub or swimming pool. In moderate climates, such as here in coastal California, flat plate collectors perform better overall because winter temperatures usually aren't that cold for extended periods.

Generally speaking, direct systems are not recommended in areas where there is any tendency to freeze during the winter. This includes areas such as coastal California, since we do get freezing temperatures at night. The thermosyphon and ICS systems are less vulnerable to freezing because they keep more hot water in the collector or near it where it can be recirculated to inhibit freezing, but in a really hard freeze there may not be enough water to recirculate. The drainback systems can freeze if the power goes out. If the direct system is only used during summer and completely drained in autumn before the first freeze, they won't freeze, but in general, an indirect system is better if freezing temperatures occur.

Solar hot water systems require some kind of backup gas or electric water heating for when the sun doesn't shine and in winter when there may not be enough sun to fufill hot water needs. One common way to install such systems is to make the solar hot water system a preheater that heats water prior to entering the backup heater tank. Another way is to build an electric or gas heater into the solar hot water storage tank, or have a separate heat exchanger coil in the solar hot water storage tank through which a heat exchanger fluid from an external boiler, like a gas boiler or a geothermal heat pump, can be circulated. A final way that doesn't seem to be very common but seems sensible is to have an on-demand electric or gas heater provide additional heat to the solar heated water if it is not hot enough.


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