Solar DHW + Space: Sizing the System

This section covers how to go about determining how much solar collector area your system should have, and what the size of the heat storage tank should be.

 

 

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This system is designed so that it can provide both space heating and domestic water heating using the same collectors and storage tank.  The sizing for space heating and for domestic water heating are different, and will be covered separately, followed by some notes how how you might combine the two sizes.

 

Sizing the System For Space Heating

Sizing the collectors and storage tank for space heating depends on several things:

 

I've just recently added a method that lets you work through all this, and make a reasonable estimate of how big your solar system should be.   But first, just to get a feel for the problem, lets consider to extremes cases at opposite ends of the scale. 

The first would be the case of retrofitting solar heating to an existing home in a cold climate.   Lets assume the home was built in the 80's and has typical insulation and sealing for that time -- maybe R11 walls, R30 ceilings, ordinary double glazed windows.  

 

The second would be a new house built to something like Passive House Institute standards.  These homes are very well insulated and very well sealed.  The

In the first case, you are very likely to find when you work through the sizing, that 1) you want to first work on your homes insulating and sealing, and window heat loss (like thermal shades).  The reason being that it will be cheaper and easier to first lower your heat loss and then add solar heating than to try to meet a very high heat demand with solar heating. 

 

Even after you have cut back the home heat loss (something that will pay big dividends in itself), its very likely that you will find that its not practical to provide a large fraction (say 90 to 100%) with solar heating.  Its likely that this would take so much collector area that either space limits or cost of system limits would keep you from getting to very high solar fractions.  This does not mean in any way that it does not make sense to add a solar heating system.  It just means that you should add a solar heating system that fits in your space and in your budget, and take whatever solar fraction this gives you.  The system will still save you lots on heating bills and will reduce your greenhouse gas emissions substantially.  Such a system will actually have a better pay back than the solar system for a very well insulated home like our second case.  I don't want to over generalize, and you want to work through the numbers on your house, but its probably reasonable to target a solar fraction in the 30 to 70% area for this case depending on your exact situation.

 

In the second case, a home that meets the Passive House Institute Standard in a northern US climate is likely to have something like R40 walls, R60 ceilings, R5 triple glazed windows, very tight sealing, and tight control over thermal bridging in the structure.  If circumstances allow, the home would also likely have some passive solar gain from south facing glazing.  Such a home would also have a Heat Recovery Ventilation system that provides good ventilation without much heat loss.  This home might have a tenth of the heat loss of our first example. 

 

For this case, it is certainly practical to target quite a high solar fraction -- perhaps even a 100% solar fraction.  The heat demand is so much lower that a much smaller collector area will meet the heat demand.  One could even experiment with backing off some on some of the Passive House Institute requirements, and trade that against the cost of a larger solar heating system to see which is more cost effective. 

 

As an example, ...   -- do a PHS house example...

 

Making the Estimate

At this point, you want to go to the page that provides the method to dete

 

Working through the details of the method will give you:

- An estimate of the heat loss for your house, and an understanding of where the heat is going.  This will be very helpful in figuring out what changes you might want to make to make the house less of a heat leaker.

 

- An estimate of the solar heat you can collect for the collector array size you enter. 

 

You can play around with making improvements to your home heat loss and changing the size of your collector array until you get to a point that looks like a good and cost effective solution.

 

 

 

 

 

Some truths: