Solar Water Heater System
Why Solar Water Heating?
Water heating accounts for a substantial portion of energy use at many residential, commercial, industrial and institutional facilities. Solar water heating systems, which use the sun’s energy rather than electricity or gas to heat water, can efficiently serve up to 80% of hot water needs – with no fuel cost or pollution and with minimal operation and maintenance expense.
Solar water heating systems can be used effectively throughout the year at facilities that have an available installation area with un-shaded / little shaded for solar collectors. A variety of building types can take advantage of solar water heating systems including residences, hotels, hospitals, school dormitories, prisons, factories. Solar water heating system are the most cost effective for facilities with the following characteristics:
- Water heating load is constant throughout the year (best with no absence in summer)
- Water heating load is constant throughout the week (will use solar hot water every day)
- Cost of fuel used to heat water is high (electricity, gas, diesel etc.)
- A clear sunny blue sky (helpful but not necessary clear, overcast is OK)
If you are interested in a solar water heating system, please don’t hesitate to fill in the Solar Water Heating Project Form and send back to us for a Solar Water Heating System Proposal. Please email to info@gmohotwater.com
What is Solar Water Heating?
Solar water heating is using a solar thermal collector to convert solar energy to heat energy, which can heat the water instead of using conventional electric or fuel energy. It makes use of the free solar energy, and also contributes to a green environment.
Active vs Passive Flow
A solar water heating system can be active or passive. This term relates to the movement of the heat transfer liquid through the solar collector. Passive systems rely on the thermosyphon action to slowly move the water through the collector. Active systems utilize a circulation pump to move the liquid. This page focuses on active systems as most large commercial systems will be designed in this format.
A typical passive solar water heating system incorporates:
- Solar collector/Collectors
- Circulation pump/Pump Station
- Storage tank
- Controller
- Piping and various valves
There are many different configurations possible, the choice of design will depend on the purpose of the system, climate, space limitations and many other factors. Lets start by having a look at the basic kinds of systems configurations.
Solar Water Heating System Types
Direct, Closed Loop & Drainback
There are three main categories of plumbing format for solar thermal systems.
- Direct
- Closed Loop (Indirect)
- Drainback
Let’s review each one by one, with a summary overview at the end of this page:
- Direct System
Suitable For
- Moderate climates (winter night temperatures > 10℃ / 50°F)
- Low water hardness (mineral content), otherwise scale will form in the collector
- Domestic or commercial applications
Design Details
- In a direct open flow system, the potable water is circulated directly through the collector. No glycol (anti-freeze) is used in the system.
- Mild freeze protection is provided in the form of pump circulation when the collector temperature approaches freezing temperatures.
- All system components in contact with the water must be potable water rated.
Advantages
- Efficient heat transfer as no heat exchangers are required.
- Allows simple retrofitting of existing hot water tank.
Disadvantages
- Not normally suitable for very cold areas. (certain system designs may allow direct flow to work in cold climates).
- Not suitable for areas with particularly hard water as limescale will form in the solar collector.
System Diagram Example
Below is an example of a direct flow system. This system has a middle mounted electric element, so the tank is accepting both solar input (bottom) while the top half of the tank is boosted as required (or with timer) to ensure hot water supply. This kind of system is suitable when there is not sufficient space for a dedicated solar pre-heat tank, or if the tank is large capacity.
The water is drawn directly from the bottom of the tank, through the collector and returned to a position about 1/4 the height of the tank.
2. Closed Loop
Suitable For
- Cold climates with winter night temperatures < 10℃ / 50°F.
- Poor water quality that would cause corrosion of the copper, or scale formation in the collector.
- Domestic or commercial applications.
- Commercial applications when final liquid to be heated is not potable water.
Design Details
- The liquid circulated through the collector is separate from the water in the main hot water system.
- The liquid can be plain water, or a glycol-water mix for areas that need freeze protection.
- The heat transfer between the closed loop and main system water is achieved via a heat exchanger, either internal coil inside the storage tank, or external heat exchanger.
Advantages
- Design to withstand freezing temperatures.
- Can protect solar collect from scale in areas with poor water quality.
- Suitable for commercial style applications such as spas, pool heating, industrial heating etc.
- Can withstand stagnating conditions without heat dissipation if designed for steamback operation.
Disadvantages
- Not as efficient as direct flow system, due to the need to use a heat exchanger.
- In cold regions, anti-freeze liquid must be inspected and changed periodically (every 2-5 years). Warm regions can just use water.
- System cost is higher due to need for:
- Heat transfer fluid (normally propylene glycol based anti-freeze)
- More complex pump station (fill & drain valves, pressure relief valve, air separator etc)
System Diagram Example
Below are examples of two indirect systems, on the left with a coil heat exchanger, on the right using an external heat exchanger.
A coil heat exchanger is normally the best choice for domestic applications. An external heat exchanger may be chosen for a domestic application if the existing tank is to be used, thus avoiding the need to purchase a dedicated tank. Commercial applications also commonly use an external heat exchanger as they can be easily sized to meet the heat exchange demands.
A disadvantage of an external heat exchanger is the need for a pump on the hot water tank side to circulate water through the heat exchanger whenever the solar pump is operational. This adds costs to the system and also additional electricity usage.
3. Drain back
Suitable For
- Cold climates with winter night temperatures < 10℃ / 50°F.
- Poor water quality that would cause corrosion of the copper, or scale formation in the collector.
- Commercial applications when final liquid to be heated is not potable water.
- System where overheating is a potential issue.
- Domestic or commercial applications.
Details
- Heat transfer liquid drain from the collector and flow and return lines whenever the pump is turned off, hence the name “drain back”.
- As the collector is empty when the pump is off, there is no water to freeze (when cold) or turn to steam (when hot).
- A high head pump is required to overcome the vertical lifting of water to the height of the collector.
- A small drainback tank is required with the water returning from the collector delivered above the water level, allowing air back up the line each time the pump turns off.
Advantages
- Prevents freezing issues, even during power outages.
- Prevents excessive heat related issues.
- Avoids the need for glycol-based heat transfer fluid.
Disadvantages
- Not as efficient as direct flow system, due to the need to use a heat exchanger.
- Piping collectors must be slightly sloped to facilitate complete drainage, this increases the requirement for the installation quality, and required straight hard metal pipe rather that flexible line sets or soft copper.
- A more expensive higher head pump is required which may use 2-4 times as much electricity as a direct or indirect circulation pump for equivalent number of collectors and pipe run length.
System Diagram Example
Below is a simple drain back system diagram, showing the small drainback tank. The heat exchanger can be either a coil in the drainback tank, coil in the main tank or an external heat exchanger (for commercial projects). Note the simplicity of the “plumbing” equipment. All that is required is a circulation pump and check valve plus some isolation and drain valves.
Which is the best system? Which is the best for a specific application?
Each system format has its advantages and disadvantages. Each can work very effectively and reliably if designed correctly and installed by an experienced contractor. The decision of which format is suitable is subject to the specifics requirements of the application. We are able to provide advices for a solar water heating system design based on years of experience. Please feel free to leave an inquiry!
Here is a summary table of the 3 system formats:
=Poor, Not Suitable, High Cost
=Excellent, Very Suitable, Low Cost
