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Sun shade reduces solar gain in Mexico (C) Daniel FriedmanPassive Solar Heating System Performance Evaluation

  • PASSIVE SOLAR HEAT PERFORMANCE - CONTENTS: How to accurately measure passive solar heating gains in buildings. Evaluating effect of infiltration losses on passive solar heat performance. Evaluating the impact of incidental solar gains on passive solar heating. systems. Passive solar heating design details. Can we accurately estimate building heat loss by turning off the heat, or measure heat gain by turning off the air conditioning system, waiting an hour, and reading the thermostat? Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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How to evaluate the performance of passive solar heating systems:

This article discusses how to make accurate measurement of the performance of passive solar heating systems, and the effect of air infiltration and the effect of incidental solar gains on passive solar systems. References to texts and guidelines for sizing thermal mass and using thermal mass are included.



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Passive Solar Heat Performance Evaluation: the Impact of Air Infiltration & Incidental Solar Gains

This article discusses how to accurately evaluate the performance of passive solar heating systems and the impact of air infiltration or incidental solar gains on passive heating performance. This material is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. The text below paraphrases, quotes-from, updates, and comments an original article, "Remember Thermal Mass?" (see links just above) from Solar Age Magazine and written by Steven Bliss.

How to make a more accurate estimate of the performance of passive solar heating systems

Question: The article entitled "The Best Passive Heating Data Yet" (Solar Age 7/83) seems to be an accurate overview of the Class B monitoring program. However the article does not clearly explain the limitations of the method of deriving the passive solar contribution to building energy demands.

Air infiltration rate skews passive solar gain estimates: While the auxiliary and internal gains of the passive solar heating system are directly measured, the solar contribution is arrived at indirectly by a subtractive technique. There is one potentially big source of error in this technique for estimating passive solar performance, namely the building's air infiltration rate. [See BLOWER DOORS & AIR INFILTRATION for added details about measuring a building's air infiltration rate.] Any error in estimating the air infiltration rate shows up as an error in the estimate of passive solar heat contribution.

Incidental solar gains can skew passive solar gain estimates: Incidental solar gains are another source of uncertainty in estimating the contribution of passive solar heating systems (or cooling systems) to a building's energy use. These include gains through non-south apertures (windows and doors) and the solar heating effect on conduction loads of the building's walls and roof.

For this reason, it would have been interesting to have included a few non-solar homes in the Class B program as controls. -- A.L., Madison WI.

Answer:

According to Joel Swisher at SERI, the one-time air infiltration measurements made concurrently with the coheating procedure were used to separate out conductive losses from air infiltration losses to obtain the building heat loss coefficient. The overall losses due to air infiltration over the heating season are extrapolated from the blower door and tracer gas results and corrected for average monthly wind speeds.

As for the incidental solar gains, Swisher agrees that this presents a problem but that achieving true scientific controls is not a realistic goal, particularly in inhabited homes. In the 1982-83 season Class B study, SERI monitored non-solar homes for comparison purposes. When the subtractive methodology was applied to these homes, solar gains in the 5 to 20 percent range were found.

This would indicate that some of the poorer performing solar homes monitored are not doing much better than a non-solar home, which is likely to be the case.

Question: Commodor 64 Solar Software

Were there any heat loss and heat gain calculation programs for superinsulation design that ran on a Commodore 64 computer back in the 1980's? - Ed Bond, Washington MA

Answer

Most software for passive solar design calculations in the 1980's would work just fine for superinsulated houses. Of the 50 programs for solar calculations listed in the 1985 Spec Guide, five were heat loss/heat gain programs that would run on the Commodore 64. They were available from Compusolar (Jasper AR), and Solarcon (Ann Arbor MI).

Another possibility in that era was Canada's HOTCAN program, devised specifically for highly insulated, tightly-sealed houses. It was available from Hotcan Energy Software, Ottawa, ON, Canada.

House Heat Loss Guesstimating by Turning Off the Heat ?

Several readers have asked why we can't just turn down the heat, wait an hour, and observe the new temperature in a building to form an estimate of the building's rate of heat loss. This is an experiment worth performing, if simply to form a quick subjective view of how quickly a building cools off on a cold day. But there are some serious inaccuracies in the "just turn off the heat and wait" approach to estimating building heat loss.

Here are some things that would be missing from this experiment, and some of these factors are major influences on the variability of a home's rate of heat gain or heat loss. Just turning down the heat and measuring temperature loss in a building fails to measure, estimate, or account for these varying conditions:

Overall it makes sense to have a general idea how a house behaves, such as from the simple "turn down or off the heat" experiment, but you cannot accurately characterize a building's rate of heat loss, nor can you know just how leaky it is, nor will you know where the major sources of heat loss are, with just the simple test of turning heat off and measuring temperature change at an arbitrary time.

These difficulties lie behind other efforts to characterize homes and their energy efficiency. See ENERGY SAVINGS in BUILDINGS for our collection of energy-savings articles for buildings, and also see SOLAR ENERGY SYSTEMS and if you are using wind power, see WIND TURBINES & LIGHTNING.

When the object is to save energy in the form of heating or cooling costs, attacking the major points of un-wanted heat loss (or gain in a cooling climate) are likely to be most cost-effective.

Some thoughts on setting priorities of attention are at ENERGY SAVINGS PRIORITIES.

Original article

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

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