Subject: Re: help please - black tiles on roof- can this heat water

From: [email-masked] (Dale C. Jones)
Date: Mon, 16 Sep 1996 14:09:05 GMT

Andrew & Jane Renouf <[email-masked]> wrote:

>I have a property with a black  roof constructed of thin slate.   On 
>sunny days the whole loft space above the house heats up to very high 
>temperatures.  (You can't stay up there for any length of time).

<snip>

>I am located just south of Great Britain.   The roof is is quite large so 
>the size of the "collector" could be very large ( 10 meters by 6 
>meters)  to make up for the poor efficiency.    

>Any help welcome.

>- Andrew Renouf

Not being rude, but isn't it exceedingly wet just south of Great
Britain?

-- [email-masked]

---

From: Andrew & Jane Renouf <[email-masked]>
Date: Sun, 15 Sep 1996 22:00:56 -0700

I was thinking  of running a long length of copper pipe directly behind 
the slates and placing insulation behind the copper pipe.

What efficiency would I get ?  How close should the copper pipes be?  I 
know efficiency would be very low,  however, we are only talking about 
the cost of the copper pipe.   (I am insulating the roof anyway as part 
of a loft conversion).

can anybody work out if it is likely to be cost effective ?

The angle of the roof is 42 degrees with a south facing aspect.

Any help welcome.

- Andrew Renouf

---

From: [email-masked] (Nick Pine)
Date: 17 Sep 1996 10:40:06 -0400

>>temperatures.  (You can't stay up there for any length of time).

Sounds promising.

The water heating efficiency would depend on the average water temperature,
Tw (F), the outdoor air temp, Ta (F), windspeed v (mph) as well as solar
intensity (300 Btu/ft^2/hr peak), sideways thermal resistance of the roof
(0.2xsx12/t F-hr/Btu, where water pipe spacing s and masonry roof thickness t
are in inches?), and the details of how well you attach the water pipes to
the roof. If they are buried in mortar, that might be pretty good thermal
connection, compared to the roof resistance itself.

You might analyze this electrical model for a square foot of roof,
to estimate the efficiency:

            ----------------- 
       ---| 300 Btu/ft^2/hr |--->---
      |    -----------------        |
      |  solar current source       |
      |                             |         I -->
 Ta  -------------www--------------------------www------  Tw
 60F?             Ra                Tr         Rr       | 80F?
                = 1/(2+v/2)                  = 2.4s/t   |
                                                     -------
You might also measure the roof temp Tr, as is,        ---
with no heat flowing into water.                        |
                                                       ---
                                                        -
The efficiency might be something like I/300. It would be low. A layer of
plastic glazing over the roof would improve it, or a transparent roof with
a dark colored attic and some fin-tube pipe running along under the ridge,
in which case you might also blow down some warm air from the peak through
a duct into the house for space heating, with a motorized air return damper. 

Nick