Solar Water Heating Systems

Benefits

Reduced Energy Consumption

  • Uses the sun’s energy to help heat water.
  • Helps reduce demands on energy infrastructure.
  • Can also contribute to space heating where hydronic heating systems are used.

Reduced Energy Costs

  • Reduces hot water-related utility bills.
  • Produces between 50% and 70% of household hot water needs when properly designed, installed, operated and oriented.

Reduced Environmental Impact

  • Reduces pollutant emissions associated with fuel-fired water heaters and electricity generation.
  • Reduces depletion of non-renewable fuel resources.

Figure 1 — Evacuated Tube Solar Hot Water System

Evacuated tube solar hot water heating system on a house roof.

Description

  • A solar domestic hot water system (SDHW) gathers solar energy in a collector and uses it to heat water.
  • A SDHW system typically consist of a rooftop collector, liquid that is pumped through it to collect the heat (water or a glycol solution), a storage tank to hold the heated liquid (usually located inside the home), a heat exchanger to transfer heat from the liquid storage tank to the domestic hot water, and associated circulation pumps, piping and controls. There are two types of collectors — flat plate and evacuated tube (Figure 1).
  • The collectors for evacuated tube solar hot water systems consist of a number of evacuated glass tubes that collect solar radiation to heat liquid flowing through pipes running inside the tubes (Figure 2). Such collectors can heat liquids to temperatures of 77°C to 177°C (170°F to 350°F). Evacuated tube solar systems can offer high efficiencies even in cold weather. However, they can be more prone to snow build up than flat plate collectors, depending on the location and the installation.
  • The collectors for flat-plate solar water heating systems consist of an insulated box with a transparent cover and a dark absorber plate through which liquid-filled pipes pass. Flat plate collectors heat the liquid up to 82°C (180°F) as it flows through tubes in or adjacent to the absorber plate. Very cold weather can reduce the performance of flat plate collectors and attention must be paid to mounting and installing them in areas with high wind loads.
  • There are two types of common systems in Canada for circulating the heating liquid from the collectors to the storage tank in the house and back. One is a closed loop system and the other is a drainback system.
  • In a closed loop system the heating liquid is in constant circulation through the solar loop and collectors at all times. In a closed loop system there is not repeated exposure to oxygen, which can be corrosive.
  • In a drainback system the heating liquid is drained to a storage tank when the system in not operating. The advantage of this is limited risk of freezing or overheating during extreme temperature conditions.

Figure 2 — Flat-plate Solar Water Heating System

Flat-plate solar water heating system on a house roof.

Figure 3 — Evacuated Tube Solar Hot Water Panels Mounted on the Steel Roof of Now House®

Evacuated Tube Solar Hot Water Panels Mounted on the Steel Roof of NOW House

Design/Installation/Operation/Maintenance Considerations

  • Professional installation is required to meet CSA F-379 — Packaged solar domestic hot water systems (liquid-to-liquid heat transfer) requirements.
  • Solar hot water system collectors are best located on a south-facing, sloped roof that is free from shading from trees, neighbouring buildings, and adjacent structures.
  • Systems located on ground-mounted racks can be easily serviced and kept free of ice and snow.
  • New houses can be designed and plumbed to be "solar ready" making it easier and less expensive to add a system in the future.
  • Utility rooms need to be large enough to accommodate the water storage tanks, pumps, piping, and controls. Ideally the collectors are located close to (or directly above) the utility room to minimize the length of pipe runs. This helps to increase the efficiency of the system by reducing the amount of energy required for moving the heat transfer fluid between and through the collectors and the storage tank.
  • Installation of rooftop solar systems is best timed with installation of roofing material. This can help reduce the need to remove the solar system when re-roofing is required.
  • A height restriction variance may be required if roof-mounted arrays have to be installed on a rack at an angle that would result in the top of the array being higher than the allowable building height.
  • Evacuated tube systems are more susceptible to snow cover in colder climates compared to flat plate systems, which generally give off enough heat to melt and clear snow from the collector area.
  • Not all collectors are suited to a drainback system.
  • Drainback and closed loop flat plate collector systems have slightly different maintenance needs:  
    • Closed loop systems use a refrigerant (glycol) as the heat transfer fluid. It needs to be drained and replaced every 5 – 10 years as it degrades over time.
    • The heat transfer fluid in a drainback is usually water that will not need to be fully drained but will need occasional top-ups.

What Does it Save?

The actual cost savings provided by a solar hot water heating system are dependent on energy costs, location, system size, collector type, storage volume, water heater type and household water usage. Households with more hot water use will see more cost saving benefits, generally.

The energy savings possible for a family of four living in a typical early 1970’s 2-storey house equipped with either a flat tube or evacuated tube solar water heating systems is shown below for several urban areas across Canada. 

In both cases, the collectors are assumed to face south and are set at the latitude angle. They are both closed loop systems. There are no obstructions and no snow cover. There is a total of 240 L of storage. The example family uses 225 L/day total hot water at 54°C (130°F).

The example shows potential savings for the house with a 58% efficient gas-fired hot water tank, a conventional electric water tank, or a 78% efficient oil-fired tank. The annual estimated cost savings associated a flat plate collector system  in Toronto range from $116 (gas-fired water heater back up) to $361 (electric water heater back up).

Annual Energy Cost Saving — Flat Plate Collector

Annual Energy Cost Savings — Flat Plate Collector
  Vancouver Calgary Toronto Montreal Halifax Whitehorse
Gas 114 105 116 164    
Electricity 233 373 361 179 382 288
Oil     356   319 340

Annual Energy Cost Saving — Evacuated Tube Collector

Annual Energy Cost Savings — Evacuated Tube Collector
Gas 111 90 107 148    
Electricity 186 316 320 159 338 263
Oil     327   286 252

*based on 2012 fuel costs

The information contained in this publication represents current research results available to CMHC. Readers are advised to evaluate the information, materials and techniques cautiously for themselves and to consult appropriate professional resources to determine whether information, materials and techniques are suitable in their case. The text is intended as general information only and project and site-specific factors of climate, cost, aesthetics, practicality, utility and compliance with applicable building codes and standards must be taken into consideration. A number of assumptions were applied with respect to fuel prices, water rates, costs of materials, equipment and labour, planning horizons, etc. Actual reductions in energy consumption and fuel savings will vary. Any reliance or action taken based on the information, materials and techniques described are the responsibility of the user. CMHC accepts no responsibility for consequences arising from the reader’s use of the information, materials and techniques herein.

Last revised: 2013

Canada

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