[bldg-sim] suggestions other than eQuest

[Steven Gates]

Lee,

OK, the rockbed is being charged with warm air from inside the house; not
from a solar air heater which could produce air at a considerably warmer
temperature.  If the maximum space temperature you are comfortable with is
80F, and the rock bed must be heated to at least 75F to later produce 70F in
the space, then we are talking about a 5F MAXIMUM temperature differential
between the rock bed and the space.

While I make my living by writing energy simulation software (DOE-2), I
believe there are many times when a full-blown computer simulation is not
justified.  So, before getting bogged down in trying to model a rockbed
system in a detailed energy simulation program, it might be useful to first
look at some rough numbers, and make a decision as to whether the rough
numbers justify a more detailed analysis.  

So, for what it is worth, here is my 15 minutes of thinking:

To move 1000 cfm at 1" static will require about 0.25 kW of fan energy. But
the air has to first be used to charge the rockbed, and then again to
discharge it.  So double the fan energy to 0.5 kW/1000cfm of heated air
delivered to the space.  At $0.10/kWh, this amounts to $0.05/1000cfm per
hour.

The amount of heat that can be delivered per 1000 cfm is:  1.08 * 1000cfm *
5dT = 5400 Btuh.  Adding in the fan heat, the net heating effect is 5770
Btuh.  At an equivalent 80% furnace efficiency and $1.00 per therm of
natural gas, the gross savings in natural gas are $0.072/1000cfm of heated
air delivered to the space.  Assuming the system can provide 100% of the
annual heating loads, the savings on the heating bill are 1 - 0.05/0.072 =
~30%

The location described gets a lot of winter sun, which even in a
conventional design will act to significantly reduce heating loads.  What
would the house cost to heat if intelligently designed (well insulated,
reasonable south glass, some thermal mass), but without the rockbed? $1500
per year?  What does the rockbed cost, $20,000?  Does a potential ~$500 per
year savings justify the cost of the rockbed?  

Of course, these numbers are very approximate, and the actual savings may
vary due to a variety of factors:
1)  If natural gas is not available, then the system may be more attractive.
2)  The 5F delta T may be too large.  80F space temperature in a cold, dry,
5000 ft desert climate may result in extremely low humidity in the space.
This may tend to desiccate the occupant's skin and sinuses, and they may not
find it comfortable.  
3)  Many hours, the fan will be charging the rockbed with the house being
cooler than 80F.  So effectiveness during those hours is reduced. 
4)  Rockbeds tend to be dusty, and may become moldy.  You may need to use a
high efficiency filter, which may raise the required fan static to do the
job.
5)  This quickie analysis assumes that the solar system can provide 100% of
the annual heating; some auxiliary heat will almost certainly be required.
6)  All losses from the rockbed to the ground are neglected.
7)  The bright sun will act to fade the furniture more rapidly than in a
conventional house.  What is impact on the furniture replacement budget?
The sun may also degrade the finish on cabinetry more rapidly, especially if
varnished.  Deduct maintenance costs from the energy savings.

For many experienced people using this listing, the above may come across as
too preachy.  To those people, I apologize.  But I understand that a lot of
graduate students also subscribe to this listing, and I want to emphasize
that considering too many details when first approaching a problem can get
in the way of the answer.  It is often useful to first look at the forest
before focusing on the trees........

Lee, I would be interested in knowing what your ultimate conclusions are.

Steve

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com] On Behalf Of Lee Elson
Sent: Monday, April 04, 2005 4:18 PM
To: bldg-sim at gard.com
Cc: bldg-sim at gard.com
Subject: [bldg-sim] suggestions other than eQuest

First, let me say that I've been a bit overwhelmed with the quality (and 
quantity) of responses. I appreciate all the discourse and am struggling 
to check out all the suggestions.

It is a good idea for me to address some of the issues I described 
originally. "ICF" stands for insulated concrete forms which consist of a 
foam form (about 2" thick) into which concrete (9-12" thick) is poured. 
These will be used for exterior walls for the single story structure. 
Although the insulating (sound and heat) properties are great, and the 
costs are reasonable, there is little heat storage by the concrete 
because of the interior foam.

Some details on the rock floor. It will be a layer of 2" rocks, about 4 
feet deep. There will be insulation underneath and a concrete and tile 
floor above. Part (about 1/3) of the tile floor will be in direct sun 
when conditions allow. There will be a couple of plenums (consisting of 
stacked concrete blocks with spacers near the bottom) running the length 
of the rock floor and there will be an air barrier of some sort that 
will channel warm air from the plenum to the outer portion of the rock 
floor. Other plenums near the ceiling will allow warm air to be forced 
(with fans) down to the rock floor where it will circulate back into the 
house through floor registers (after having given up heat to the rocks). 
This rock floor will be used over the entire structure (about 3500 sq 
ft). The builder/designer says that he's used this approach and it works 
pretty well. There is sufficient heat transferred to the rocks to make 
them a reasonable thermal mass. This system also redistributes air from 
the warm (south/high) side of the structure to the cold (north/low) side.

I realize that this type of thermal mass may be difficult to model in 
detail and that it is important to look at the energy cost of the fan(s) 
in all of this.

I hope this description helps.

Regards,
Lee Elson

Steven Gates wrote:

>Is there a more fundamental issue that is being missed here?  Lee Elson's
>description includes the phrase "insulated rock floor with active air
>recirculation".  It also includes "ICF", which I am not familiar with.
>
>The description of the system appears to be a floor/rock-bed that uses a
fan
>to recirculate air between the rockbed and the space.  So solar falls
>directly on the floor surface, but the floor also captures/releases heat
>convectively using a fan.  
>
>If that is the case, it is critical that the convective leg be simulated.
>Assuming the space will not be allowed to be uncomfortably warm, and that
>the rockbed temperature must be at least 70F to have any heating
>effectiveness at all, this implies a very low delta-T between the rockbed
>and the space when convectively charging or discharging the rockbed -
>corresponding to a large, potentially drafty, airflow.  It also suggests
>that the convective thermal capacity of the rockbed may be quite limited,
>corresponding to a temperature swing on the order of 5F or less.  
>
>It is essential that the fan energy of this system be modeled; with such a
>small delta T between the rockbed and the space, the cost to run the fan
may
>exceed the cost of the heating energy saved by the convective component of
>this system.
>
>It would be helpful if Lee described the system in more detail.
>eQUEST/DOE-2 does not simulate either convective rockbeds or the combined
>radiant/convective rockbed floor that this building appears to use.  Do any
>programs have this capability?  
>
>
>-----Original Message-----
>From: postman at gard.com [mailto:postman at gard.com] On Behalf Of Blake, Jeff
>Sent: Sunday, April 03, 2005 8:03 AM
>To: bldg-sim at gard.com
>Subject: [bldg-sim] suggestions other than eQuest
>
>Lee,
>
>Doe2 (2.1 or 2.2) is not the right program for looking at heavy thermal
mass
>buildings.  The limitation is a function of the thermal response factors
>used to represent time-delayed heat transfer through opaque surfaces.  This
>method is a legacy of the original DOE2 design requirements which were
>somewhat hindered by slow computers.
>
>You should use a simulation program that uses a more fundamental heat
>balance approach.  There are several but I believe that ESP-r (ESRU,
>University of Strathclyde) is one of the best.
>
>Jeff Blake
>
>-----Original Message-----
>From: postman at gard.com [mailto:postman at gard.com]On Behalf Of Lee Elson
>Sent: Friday, April 01, 2005 3:55 PM
>To: bldg-sim at gard.com
>Subject: [bldg-sim] suggestions other than eQuest
>
>
>I'm designing a 3500 sq foot residence at a 5000' desert-like elevation
>(Nevada).
>The site has good sun exposure and the climate is generally dry. Typical
>temperatures in Feb are 20-40 F. The building is mostly oriented east-west
>and has
>a 2' deep insulated rock floor (for thermal mass) with active air
>recirculation.
>ICF's will be used as well as tile over concrete (above rock floor) in
south
>facing rooms.
>
>I'm trying to get a realistic estimate of temperature swings and auxiliary
>heating
>requirements. I've used eQuest, which seems pretty impressive and easy to
>use.
>I've entered the floor plan layout, specified the glazing (clear glass on
>the
>south side) and the building materials. The calculations are a bit
>disappointing:
>with 12% of the total floor space in south facing glass, the aux heating
>required
>is not too different (~20%) from a house with standard insulation and no
>added
>south facing glass. Another oddity: when I change the south facing glass
>area from
>5% of the available south wall to 90% of the south wall, I get an
*increase*
>in
>aux heating requirements. The eQuest developer thinks this is due to
>inadequate
>thermal mass/heat recirculation modeling.
>
>I suspect that eQuest is not doing accurate modeling since other houses
with
>a
>similar design in this area get much better thermal performance. Can anyone
>suggest either software or a service provider that might be able to do a
>more
>accurate calculation?
>
>TIA
>
>Lee Elson
>
>
>
>
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