Exploring the Advantages and Challenges of Drainback Solar Water Heaters

Drainback solar water heaters are a smart way to use the sun's energy to heat water, which is great for the environment and can also save money.

But, like any technology, they have their upsides and downsides.

In this discussion, we'll take a closer look at how these systems work, what makes them beneficial, and some of the challenges they might bring. We'll talk about why these heaters can be a good choice for your home or business, but also why they might not be perfect for everyone.

Our goal is to give you a clear picture of drainback solar water heaters, so you can decide if they're right for you.

Drainback solar water heaters: What is it?

A drainback solar water heating system is a type of solar thermal system designed for heating water. It's unique for its use of a "drainback" reservoir to protect the system from freezing or overheating.

This design ensures that water is not left in the collectors when the system is not in use, thereby reducing the risk of freezing or overheating. It is known for its reliability, simplicity, and low maintenance.

However, while it offers significant advantages, implementing a drainback solar water heater is not the simplest task. It requires specific knowledge, time, and a financial investment for proper design and construction. Cost-wise, it sits between the extremes, being less expensive than the closed-loop system that uses antifreeze (propylene-glycol based active type), but more costly than the Integrated Collector Storage (ICS) system.

In a drainback system, the risk of boiling water, high pressure, freezing, and bursting pipes is minimized, as long as the down-flow pipe is large enough and the fluid loop in the solar collectors and pipes is adequately sloped for draining. This technology employs water as the heat transfer medium, which, unlike heating fluids with additives such as antifreeze, does not corrode pipes and other metal parts, nor does it deteriorate over time.

Furthermore, water is more cost-effective, has better thermal properties, and requires lower maintenance compared to other mediums.

How it works?

The drainback solar water heater system operates on a unique yet straightforward principle, which combines the use of solar energy, gravity, and a specialized tank to heat water efficiently.

Here’s a detailed breakdown of its operational process:

Solar collectors: The system uses solar collectors (usually flat-plate or evacuated tube collectors) installed typically on the roof. These collectors absorb solar radiation, converting it into heat.

Heat transfer and circulation: A heat transfer fluid, often water or a water-antifreeze mixture, circulates through the collectors. When the sun heats the fluid in the collectors, the warmer fluid is circulated through a heat exchanger in the water storage tank. This process transfers the heat from the fluid to the water in the tank, warming it up.

Drainback reservoir: What sets the drainback system apart is its unique safety mechanism. When the system is not in operation (such as during nights or cloudy days), the heat-transfer fluid drains back into a reservoir. This prevents the fluid in the system from freezing in cold weather or overheating and boiling in very hot conditions.

Control system: The system includes a control mechanism that monitors the temperature of the fluid in the solar collectors. When the fluid reaches a set temperature, a pump activates to circulate the fluid through the heat exchanger. When the temperature drops below a certain point, the pump switches off, and the fluid drains back into the reservoir.

Heat exchanger: Once heated, the fluid passes through a heat exchanger, which is typically located inside a water storage tank. The heat from the fluid is transferred to the domestic water in the tank.

Hot water usage: The heated water in the storage tank is then available for domestic use in showers, taps, etc.

Types of the drainback systems

Drainback solar water heating systems are an efficient way to harness solar energy for residential water heating. These systems typically include well-insulated storage water tanks, a small drainback reservoir, solar collectors, a closed-loop with a pump, controllers, and a heat exchanger.

There are three main types of drainback systems, each differing in their configuration and the location of the heat exchanger.

• The first type features a heat exchanger located inside the water storage tank. This design simplifies the system by using only one pump to circulate the fluid through the solar collectors and then through the heat exchanger within the tank. As the fluid passes through the heat exchanger, it transfers heat to the water in the storage tank, heating it for use in the home.
• The second type positions the heat exchanger below the minimum fluid level inside the drainback tank. This setup involves a second pump, which moves water from the water storage tank through the heat exchanger and back into the tank. The heat from the fluid in the drainback tank is transferred to the water as it circulates through the heat exchanger. This type allows for a more controlled heat transfer process and can be more efficient in certain configurations.
• The third type uses a heat exchanger located outside the water storage tank. This arrangement provides flexibility in system design and can be advantageous in terms of maintenance and system efficiency. The external heat exchanger can be easier to access and replace if needed, and it allows for more customization in the overall system layout.

In most cases, the heating fluid in these systems is water, though sometimes propylene-glycol is used. However, water is often preferred because propylene-glycol can deteriorate over time and requires replacement. Additionally, using water as the heating fluid enables the possibility of an open-loop system, particularly in warmer regions where the risk of freezing is minimal. It's important to note that open-loop systems are more prone to freezing, so their use is generally recommended in climates where freezing temperatures are not a concern.

Each of these drainback solar water heating system types offers unique advantages and can be tailored to specific needs and local climate conditions, providing an efficient and environmentally friendly solution for residential water heating.

Sizing a drainback tank

When sizing a drainback tank for a solar water heating system, it's essential to consider the total volume of fluid that will fill not only the solar collectors, but also the piping that runs above the tank.

Additionally, it's advisable to add several extra gallons to this total to ensure adequate capacity. These tanks must always be insulated to prevent heat loss, thus maintaining efficiency.

For safety and proper operation, they should be equipped with a pressure relief valve and appropriately vented. Another crucial feature is a sight glass, which allows for easy monitoring of the fluid level inside the tank.

Optimally, the tanks should be located as close as possible to the collectors to minimize heat loss during fluid transfer and to facilitate efficient system operation.

This careful consideration in sizing and placement of the drainback tank is vital for the effective and safe functioning of the solar water heating system.

The key role of circulation pumps

A drainback solar water heater might utilize either one or two pumps. In a two-pump system, one pump circulates water from the drainback tank to the solar collectors, while the other moves water from the water storage tank back to the drainback tank. The recommended types of pumps for these systems are high-head, low-flow pumps, which may be variable speed or booster pumps.

When the system is activated, the pump fills the system with fluid and then circulates this fluid throughout. Upon turning off, the fluid drains back into the smaller tank.

It's crucial for the pumps to have sufficient head pressure to move the fluid up to the collectors. However, energy consumption can be minimized by employing the thermosyphon principle for fluid circulation.

In areas with high mineral content in the water, commonly referred to as hard water, there is a risk of pipe clogging over time. This clogging can impede both the thermosyphoning process and the draining of the system.

Solar/hydraulic loop: Things to consider

All pipes in a drainback solar water heating system must have a sufficient slope, ideally between 15-30 mm per meter (1/2" to 1" per 3 feet), as the drainback process relies on gravity when the pumps stop.

It is crucial to prevent sags and pipe damage, as these can lead to incomplete draining and potential frost damage.

Selecting the correct materials and ensuring proper mounting are equally important. The recommended pipe size for such systems is typically 3/4 inch.

Highlights: Advantages of drainback systems

Energy efficiency and environmental benefits

• Harnesses solar energy, reducing reliance on conventional energy sources.
• Contributes to lower greenhouse gas emissions and a smaller carbon footprint.
• Leads to significant savings on energy bills.

System longevity and low maintenance

• Outlasts many conventional water heaters due to durable design.
• Features fewer moving parts and less corrosive materials, reducing the need for repairs.
• Offers a hassle-free, low-maintenance solution for long-term use.

Safety features

• Includes protection against freezing in colder climates.
• Prevents overheating in hot weather or during periods of low usage.
• Provides a safe and reliable solution in various environmental conditions.

Disadvantages

A significant disadvantage of drainback solar water heaters is the potential for heat loss when the system is not active, especially during night exposure. Since the system relies on solar energy, there's an inherent limitation in retaining heat without continuous sunlight. This means that during the night, or on cloudy days, the system may lose a considerable amount of the heat it gathered during the day.

Drainback systems necessitate the use of larger pumps compared to other solar water heating systems. These larger pumps are required to effectively circulate the water through the system, but their size can contribute to higher initial costs and potentially greater energy usage for operation, slightly offsetting the system’s overall efficiency.

While the concept of the drainback system is relatively straightforward, designing the solar loop requires special attention. The most challenging aspect is ensuring that all water from the collectors drains back into the reservoir when the pump stops. Achieving this efficient drainage, which depends on gravity, while ensuring air flows upwards into the solar panels and outdoor pipes, can be complex and requires precise engineering.

The presence of air in the system introduces the possibility of metal corrosion over time. This aspect is particularly crucial in drainback systems where air is a part of the solar loop. The interaction between air, water, and metal components can lead to corrosion if not properly managed, potentially impacting the longevity and efficiency of the system.

Recommendations

The drainback solar water heater is suitable for all climates, including the coldest, due to its ability to drain fluid from the collector and exposed pipes, preventing freezing damage. Therefore, it's crucial to design the system to ensure quick and complete fluid drainage.

Additionally, the tank should be installed as high as possible while still allowing for complete drainage. To facilitate this, horizontal pipes should be avoided. Instead, ensure all pipes have a minimum 15-degree slope, and opt for 45-degree fittings rather than 90-degree ones for more efficient drainage.

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