Why you're not getting the savings you've been promised from you solar water heating system?
Businesses and Homeowners frustration over failed Solar Water Heating installations (or even PV solar installations) is the primary reason for thousands of lawsuits across the US and Canadian Market. The majority of People who invest in any energy efficiency measure (including solar water heating for Space heating and DHW) are driven by a medium to long term vision for energy savings, which is expected to be translated into lower utility bill, less energy cost and more money into their pockets.
So why many Solar Installation do deliver their promised savings and why others do not? It starts by a good solid design of the solar water heating system, then qualified installer and Proper Operation of the system. All these Three Factors are important and are the 3 prerequisites for the success of any solar installation, but alone are not enough when there in no coherence among them.
This Blog will outlines the most common major causes of Solar Water Heating investments failures such as:
1- Integrated Design and the involvement of project's stakeholders in the preliminary phase of the design process.
2- Location of Makeup Heat inside the solar storage tank or outside.
3- Roof Installation Tilt Angle toward Roof Tilt Angle.
4- Fluid Heat Transfer Flow and Temperature difference between inlet and outlet of solar panel.
Integrated Design Vs Design in Silos
1Integrated Design is the back bone for every successful project. The Integrated Design Process (IDP) is a method for realizing high performance buildings that contribute to sustainable communities. It is a collaborative process that focuses on the design, construction, operation and occupancy of a building over its complete life-cycle. It also emphasis on the involvement of Client, Designer and Installer (and many other stakeholders) in order to ensure that what the project will deliver will meet stakeholders expectations.
Integrated Design Process will prevent someone from investing in Solar Water Heating system designed for in-floor space heating (usually works at 105 Degree Fahrenheit Hot Water Temperature) and use it for domestic water heating (Required to deliver Hot Water at 140 Degree Fahrenheit) and vice versa.
Another Advantage of IDP is future scale-ability and flexibility of the installation. Knowing what is required in the future, will allow designer to properly design a solar water heating system that can be scaled in the future without having to invest a penny in retrofitting the existing system.
Location in Make-Up Heating Element
2This is the reason the 60% of Solar Water Heating Systems lower than expected performance. The Majority of Solar Water Heating system sold in Canada and the Northern Part of the US are Split type pressurized solar water heaters, where Solar Panels (usually Vacuum Tube Solar Collectors) are located outdoor (whether on the roof or the back yard) and solar hot water tank with embedded electric makeup heating element is located indoor (in the basement or garage).
2.1Makeup Heating element (pointed by the red arrow in the picture on the left) is very common due to its low cost and affordability. Buying a Solar Water Heating Tank with a 1.5 KW Electric Makeup Heating element will cost the same as a Solar Water Heating Tank that has no element.
That will significantly impact the performance of every solar water heating system especially in Nordic Climates where heating demand can go significantly high during sunny winter hours.
Most Solar Thermal Storage tanks are sized to be heated in 4 to 6 hours daily sunshine. Having the makeup heating element inside the thermal storage tank, will decrease the energy efficiency of the solar thermal panels, since in high heating demand, element will heat the tank (in less than an hour) much faster than solar panels making a large portion of recovered sun's heat non usable in high demand. Also Solar Water Heating Panels thermal efficiency is inversely proportional to tank water temperature.
2.2As shown in the above schematic, in order to prevent over usage of make up heating element, we recommend installing the makeup heating device downstream the solar thermal storage tank (at the tank's outlet) and having the return of hydronic heating loop connected to the solar water heater tank's inlet. This configuration has multiple advantages:
1- Hydronic Heating Loop Return Temperature is lower than Supply Temperature. Return will be preheated by the free energy recuperated by solar panels and whatever solar can not provide will be supplied by the inline makeup heater (usually Electric, Gas or Propane tankless modulating boiler). Here we recommend a solar thermal storage tank set point temperature like 3 to 5 degree Fahrenheit above Boiler's supply water temperature. So when solar system is able to heat to the desired temperature set-point, boiler's heater will stay off.
2- As explained above, having lower water temperature in the tank, will increase the thermal efficiency of the Evacuated Tube Solar Collectors as well as the solar water heater tank lifespan.
Evacuated Tube Solar Collector Tilt Angle Vs Roof Tilt Angle
3When installing vacuum tube solar collectors on a tilted roof, and since in Canada Hot Water Heating demand is in winter time, installers shall avoid installing panels at the same tilt angle as the roof since panel will be covered in snow and ice for a good 3 to 4 months a year. For Tilted Roofs, we recommend either raising solar panel by a feet above roof or tilting the evacuated tube by like 30 degree angle from the tilted roof to avoid snow and ice accumulation on top of the panel.
The problem of snow and ice accumulation is less critical for flat installations since tubes are spaced by like an inch and snow can fall between tubes to the ground. The little ice that accumulate on the outer surface of vacuum tubes will melt after a couple of minutes of solar radiation since temperature inside Evacuated Tubes Heat Pipe can go to like 150 Degree Celsius.
Fluid Heat Transfer Flow and Temperature Difference Through Evacuated Tube Solar Collector
4Solar Thermal Heat usability is driven by not only the quantity of generated energy but by its quality. Quality of thermal heat is defined by its ability to be transferred and exchanged. Exchanging heat among fluids (Glycol to Water, Water to Air, Air to Humans, etc....) requires a temperature difference between heat supplying fluid and heat receiving fluid. Having a lot of thermal heat at the wrong temperature will make it useless.
The Application of Hydronic Water Heating defines its operating temperatures conditions (Ex: in-floor heating requires a supply water temperature of 100 Degree Fahrenheit and a return temperature of 80 Degree Fahrenheit). For any solar thermal system to be useful for the intended application, it should be able to supply its heat transfer fluid at a temperature above the operating temperatures of the end user application (such as space heating, Domestic Hot Water heating, pool heating, etc....).
Since Solar Radiations vary season to another and daily hour, Fluid Heat Transfer Flow should be properly balanced for the intended application and should have been designed accordingly. For variable temperature applications, we recommend using pumps with ECM motors and more sophisticated control.
Conclusion and Recommendations
Prior of committing of buying any solar water heating system, it's crucial to communicate to your supplier/designer the intended application of the system. Many nowadays suppliers have in-house engineers who can customize the solar water heating system for your application.
It will also be nice to ask for a life cycle analysis, since what matters is not only the initial cost (project's cost), but also the running cost of the system (Maintenance, Energy Savings, etc....). Life Cycle Analysis is the way to go.