Solar heating box

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[edit] Project Participants

Spring 2010:

[edit] Opportunity Definition

My project is to design and test a passive solar heating box. The objective of the design is to collect heat and transfer it into a house via convection.

Solar heating boxes are appropriate technology because it utilizes the resource available in an effective manner. This type of design is appropriate in almost any location, being that its means of energy are solar and natural convection. Passive solar designs can provide a comfortable environment anywhere from garages and workshops to third world villages. It has a smaller carbon footprint than conventional heating systems.

It was to originally be installed in the garage/jam space of Synrgy, reggae massive from Humboldt County. However, due to lack of solar exposure, the decision was made to test the design at another garage right down the road that serves as the jam space for local bands Acufunkture and Blue Street Junction.

[edit] Literature Review

[edit] Energy:Its Use and the Environment

This is a college level textbook that breaks down the physics of energy and heat transformation. It breaks down all of the necessary mathematical procedures on finding heating loads, specific heat, amount of solar insolation, ect.[1]

[edit] The Passive Solar Construction Handbook

This Book breaks down the necessary components of a passive solar house or heating system.[2]

[edit] Solar Energy for Heating and Cooling

This book lists the amount of solar insolation of virtually every city in America, breaking it down by the time of year, time of day and latitude. [3]

[edit] Peer reviewed Journal

A journal discussing the heating storage capacity of various building materials.[4]

[edit] Criteria

The objective of my project was to design a completely passive solar heater that is built from inexpensive and readily available materials. I came up with a criteria that would influence my design based on its ability to transfer heat, its size, cost, and aesthetics.

Criteria Weight Constraints/considerations
Generates Heat 10 Collects heat and transfers it into the room.
Passive 10 The future addition of a fan may slightly alter this criteria.
Aesthetics 5 Interior of box must be black
Cost 8 Less than ~$200 material
Heat Storage 8 System must be Closed at night/materials in room can increase heat storage
Size 9 Must be no bigger than 4X6 ft.

Criteria Weight Alt Solution 1 Alt Solution 2
Generate Heat 10 50 50
Passive 10 50 20
Aesthetics 5 25 40
Cost 8 40 30
Heat Storage 8 45 25
Size 9 45 45

  • Alt solution#2 is a energy star floor heater

[edit] Design

The Basic Design Consists of a simple frame with a heat collector centered in the box. A tempered glass cover encloses the box and creates a "dead air space" between the collector and the glass. Beneath the collector are two ducts. The cold air inlet is at the bottom of the box and the hot air vent is at the top.

[edit] How It Works

As sunlight hits the collector, it absorbs heat, heating the inside of the box. As the air in the collector is heated up, it becomes less dense and is released out the hot air duct into the room while convection draws cold air from into the box. That air is heated up and rises through the air flow passage to the hot air outlet. This is known as a thermosyphon.

The box will continue to transfer heat to the room until the temperature of the air in the box drops below the temperature of the room. As the box cools, it can create a reverse thermosyphon effect and start drawing warm air out of the room. Therefore it is necessary to close this system off at night.

[edit] Passive Solar In Arcata

Although the sunny days are quite inconsistent in Arcata, a passive solar design like this is never a bad idea. It may take up space, but it requires no energy. Sunny days usually lead to cloudless and very cold nights because all of the heat of the day gets released into the atmosphere. Therefore, sunny days are the most vital to absorb as much heat as possible.

[edit] Sizing the Heater

To size properly size a heater, one must calculate the total heat loss from the room and determine how big their collector would have to be to get the desired temperature. Calculating heat loss takes into account the size and building materials of the room. To calculate heat loss, you need to find the room's total conductive heat loss and add that to the infiltrating heat loss. It's a pretty lengthy calculation. The equation for conductive heat loss is:

conductive heat loss=(area of room)*(u-value)*(temp difference)

The infiltrative heat loss equation is:

Infil. heat loss=(volume of room)*(specific heat of air)*(temp difference)*(air change rate)

Here is a link to the step by step process of calculating heat loss. [1] Here is a link to a list of building materials and their insulation value. [2]

There are other considerations to take into account when calculating heat loss. The geographic location, orientation of the house, and number of people living in the house are all factors that influence the amount heat gained or lossed.

I calculated my total heat loss of the room to be 3405 btu/hr. It's a little tricky to size a heater to meet a home's total heating demand. One rule of thumb is to have 2 square feet of collector for every 10 square feet of room. [5] Basically, the bigger your collector, the more heat you will transfer.

Howver, due to size constraints on my criteria, I'm limiting the size of my box to 4 ft by 6 ft.

[edit] Insulation and Thermal Mass

One of the biggest components to a passive solar heating system is the insulation of the room as well as the thermal mass. Insulation keeps the heat from infiltrating out of the room. Thermal mass absorbs heat during the day, then re-radiates it at night. Night is the most important time to keep warm, so incorporating thermal mass in a passive solar design is very important.

[edit] Construction

[edit] Box

I started by building a 4ft by 6ft wood frame out of 2'x6's. Similar professional designs use aluminum enclosed frames, but wood works just fine.
Fig 1: The Frame

[edit] Vents and Air Passage

I screwed a piece of plywood cur to fit on the frame. I Placed my wood baffles, leaving about a foot gap as shown in the picture. I then cut two four inch holes for my vents. Then I lined the walls of my frame with insulating foam. I made sure to tightly seal all of the cracks and seams with caulk.
Fig 1: Baffles and Vents. Moving the top vent to the top right corner would increase air flow

[edit] Heat Collector

For my heat collector, I used corrugated sheet metal that I found at a local scrap metal yard. I cut and connected the pieces of sheet metal to fit perfectly inside the box. Using a pop rivet gun, I it the panel three inches deep in the box with metal brackets. I then painted the collector black to increase its heat absorption. I used BBQ black paint because it can with stand very high temperatures.
Fig 1: Heat Collector

[edit] Cover

The ideal cover would be a tempered glass because it is stronger and less likely to oblique from the sun. Due to budget constraints I used a 4ft by 6ft sheet of plexiglass. I placed the plexiglass on the frame with some weather stripping in between the wood and cover. I used metal braces to screw the plexiglass down. (Very carefully.) I then caulked the seams of the braces.

[edit] Orientation

The general rule of thumb is to orient your collector facing south at an angle equal to the latitude plus 15 degrees. [6] The latitude in Arcata is 40 degrees, so the orientation of the collector should be south facing at about 55 degrees.

[edit] Attaching the Heater

The house that the heater was the be attached to was occupied by renters, so we couldn't put any holes into the wall. Instead, I hung a sheet of plywood over the outside door frame of the back door of the garage (nobody uses that door). This is to be a temporary wall for testing purposes. Since I plan on putting the heater in my next house, I didn't want to permanently place this heater at the testing house. I cut two 4 inch vent holes and connected the box to the wall with aluminum ducting. On the side of the plywood facing into the room, I placed vent covers over the holes. The idea is to leave the door open on sunny days, so the heat releases into the room. At night and on cloudy days, you can shut off the system by closing the door.

[edit] Testing

Testing was a bit tricky for this room because it was poorly insulated. My main objective however, was to determine if the box could collect heat and transfer it into the room through the ducts.

[edit] Problems

The first obvious problem after installing the box was the climate. The first five day were cloudy. On the first sunny day I was surprised to see that the box didn't seem to be working. I could tell that air in the box was very hot, it just wouldn't transfer out. I decided to bring the box more parallel with the wall to shorten the vents. This seemed to work a little bit. I finally realized that the bottom vent cover was preventing the convective flow. Once I took it off, the hot air started pumping out. Air flow could still be improved by moving the hot air release vent to the top corner, so the air flow is directed right to the vent. This would prevent air from collecting above the vent, instead of circulating out into the room.

[edit] Results

At full sun exposure, the air coming out of the heater averaged 100 degrees fahrenheit. The overall room temperature averaged about 72 degrees fahrenheit at full sun exposure, which is about 7 degrees above average. Due to poor insulation, the temperature would quickly drop as the sun went down. Because the box was able to passively collect heat and transfer it into the room, it met all the components of my criteria. Therefore, this project is a success.

Fig 1: Temp of air coming out of the box

[edit] Further Considerations

To insure the longevity and durability of this project, several steps could be carried out:

To improve air flow, the top hot air vent could be repositioned to the very top corner of the box, where the baffle would direct the air right to the vent. This would improve the efficiency and performance of the box.

The box needs to be coated with a water resistant paint. It is important to prevent moisture from entering the box, which would hinder it's heating ability. This step would add to the longevity of box.

A supportive frame must be built that secures the box at the proper orientation. This would insure the durability and safety of this project.

Incorporating thermal mass in the room to be heated is a vital step that anyone must take when using solar heating systems. Perhaps hanging a thermal blanket over the vent or building a trombe wall. Incorporate

[edit] Budget

Quantity Material Source Cost ($) Total ($)
1 Plexiglass 48"x72" acrylic sheet Hensel's Ace Hardware $68.65 $68.65
2 Aluminum Duct 4" Arcata Lumber $4.10 $8.20
2 Duct Cover 4" Arcata Lumber $2.50 $5.00
2 Alum Duct Connectors 4" Arcata Lumber $2.51 $5.47
2 OSB Sheetwood 3/4" Arcata Lumber $18.00 $36.00
1 2x6x20 dougfir Arcata Lumber $9.08 $9.08
1 Can black bbq paint $5.39 $5.39
1 Corrugated sheet metal Arcata Scrap and Salvage $10.00 $10.00
2 Foam Weather Strip Hensel's Ace Hardware $2.69 $6.00
1 Misc. Hardware Arcata Lumber $10.00 $10.00

[edit] Tenative Timeline

March 15th-Collect Materials from Hardware store

March 20th-Collect Plexiglass and donated materials. Start taking measuring and taking temperature of the room. Start Calculations

April 1st-Have all calculations finished

April 2nd-Size the heat box

April 10th-Build box, Attach vents to house

April 11-15th-Test the box. Take Temperature of room.

[edit] References

  1. Hinrichs,R.A. Kleinbach,M.H.(2005). "Energy:It's Use and The Environment." 4th Edition. Brooks Cole. New York.
  2. Levy, Emanuel(1983). "The Passive Solar Construction Handbook." Steven Winter Associates,Inc. Rodale Press.
  3. Jordan, Richard (1977). "Applications of Solar Energy For Heating and Cooling of Buildings." American Society of Heating, Refigerating, and Air Conditioning Engineers, inc. New York.
  4. Kedl, R.J.(2001). "Thermodynamics of Wallboard with Latent Heat Storage for Passive Solar Applications." U.S. Department of Energy. Sante Fe, New Mexico.
This page was part of a project for Engr305 Appropriate Technology, which finished on May 15, 2010. It is now open edit.