Hot Roof Problem
We are faced with a world, regions and individual localities that are heating up beyond the range that is good for the wellbeing of the planet's ecology and our comfort and productivity. We have Global Warming, expanding Desertification the urban Heat Island? effect and the Hot Roof Problem to deal with and each can aggravate the individual situation, particularly for people living in Tropical Climates and Subtropical Climates?. Now all these problems are caused by solar energy and our lack of knowledge of how we can utilize this energy efficiently so that it is not a burden but becomes a renewable resource that enriches our lives. We are expending an inordinate amount of money and energy to cope but we are not addressing the real problem of hot roofs. We think the answer is to cool the building interior while we let the roof and building envelope reach very high temperatures.
Additionally, there are other thermal sources in buildings that will lead to overheating. This includes the wall areas, including windows and may also include air changes that bring hot outside air into our Controlled Environment buildings. The solution that Sola Roof presents is very radical but simple and it is to build and demonstrate a Cool Roof? solution: roofs which act as a thermal energy sink and will function not to overheat the building but to cool it down.
The engineering community, by following Mechanical and Air Conditioning? methods has not been able to solve this problem. However, using Bio Mimickry? methods and the Blue Green solution Sola Roof is showing the way to a future where cooling is a sustainable process that is based upon the exploitation of the cooling power of living plants in buildings and this approach naturally supports the implementation of Closed Ecological Life Support Systems within our homes and communities. These are grand visions that will radically change our world and not only that - they are easily within reach. Not only the rich nations but also poor communities can afford to get onto this path to a Sustainable Future?.
Below is a discussion between myself and Danny Parker (by email in 2004) @FSEC.UCF.EDU on the subject of Hot Roofs? and the Cool Roof? solution and I hope others will contribute to the concept development and vision. - Sola Roof Guy
I will try to be more precise in putting forward some observations for your further comments and analysis. One important question can be put: What temperature do various roofs reach at average peak midday insolation? I have experience with galvanized steel sheet metal roofs (that I believe have good reflective properties) that in the tropics become very hot. If you back them with insulation (including a simple roof cavity) they get even hotter. Traditional thatch grass roofs are substantially cooler. If you look at the temperature of living grass or sod cover (or the soil) it is substantially cooler. What is the actual peak temperature of each roof surface?
Another point worth noting is this observation:
Additionally, it is clearly pointed out that roofs with low emissivity during the heating season there will be a negative or lack of chilling effect at night and a lack of benefit of roof warming during the winter day. Even in the south the negative energy deficit compared to a more typical (darker) roof should be included in a year around energy benefit analysis. This illustrates the problem of setting fixed (passive) values in the roofing material.
Sola Roof has no fixed values and can be changed to suit the needs of the season and of the hour of the day. Our ideal specification for the Sola Roof is a roof with maximum transparency and would have about 4% reflection, 2% absorption and 94% transmission of the entire radiative spectrum (including the UV, visible, solar infrared and long wave thermal radiation). However our Liquid Bubble? technology is opaque to the infrared and thermal radiation and is used as needed for energy conservation in the form of Liquid Solar Insulation?. At other times, when roof chilling is helpful we have the option of operating the Liquid Solar process at night to radiate thermal energy to the night sky. Conventional roofs of the "best specification" but having fixed radiative properties cannot perform in all cases.
In discussing the temperature of the roof structure it is my understanding that it is the temperature difference between the ambient and the actual roof temperature that would indicate the relative efficiency. When galvanized, white, green, gray or black roofs are compared they all reach a relatively great temperature difference and in the real world demonstrate only a smaller percentage of differences between each other. Do you have data on this? Can you do a simple test to establish these facts if they are not absolutely clear?
Insulation is costly and many developing economies may not be able to afford this investment in high spec, high insulation roofs. Neither will insulated roofs lower the temperature to roofs; in fact a hotter roof surface results. Therefore Sola Roof is an opportunity for a cool roof solution has the great merit of not needing any insulation. The Sola Roof that I invite you to look into is such a roof structure. The structure of this roof remains cooler than the ambient temperature (usually referred to as the dry bulb temperature) due to the liquid-cooling process. Additionally, the chilling action on the thin Liquid Film cooling will prevent the roof from warming above the wet bulb temperature. This is a simple fact and your organization could replicate this data very easily. We also have recently established three full-scale Sola Roof projects (please see our Websites) to prove out these operational parameters.
A transparent roof constructed according to the Sola Roof concept has the unequalled benefit of providing day lighting within the structure. If we can quantify this benefit, I feel certain that the savings on artificial lighting will be an order of magnitude greater than the pumping load mentioned above that is required by the Liquid Solar and Liquid Cooling processes. This is a real advantage compared to conventional glazing systems like windows and skylights that are not liquid cooled. The thin liquid film chilling can further reduce the pumping load because the solar gain is rejected as latent heat of water vapor. I know that there will be a lot of other detailed questions, since there are several interrelated innovations involved in the total package.
Original message from Danny Parker dparker@FSEC.UCF.EDU
Richard, I agree with some of what you say, but other portions are incorrect.
1) While it is true that flat roofs or tile roofs will soil and degrade in reflectance, white metal maintains its reflectivity for years and performs quite admirably. Recent research at Oak Ridge National Laboratories verifies the above fact. http://www.ornl.gov/roofs+walls/facts/RadiationControl.htm
2) The army's experience with tent fabrics is not indicative of what happens in houses. Note that in the Ft. Myers study we looked at full-scale houses under very carefully controlled circumstances. It showed that contrary to what you suggest that reflectivity of the roofing material is THE factor influencing ceiling-roof-attic related cooling performance.
3) Evaporative cooling strategies such as roof ponds, roof sprays and "green" roofs such as what you suggest, while effective tend to be much more expensive than altering roof reflectivity. They also tend to be maintenance intensive. I would need a lot of convincing to change my mind on that because I have seen some very ingenious cooling methods cast aside due to complexity and expense. Reflective roofs are an elegant solution.
4) Savings of "green" roofs cannot be "an order of magnitude greater than the Florida Study results." An order of magnitude is 10 times the quantity. We showed that reflective roofs saved 20% of the residential cooling load. One cannot save 200% of the cooling load! My guess is that a "green," evaporative or roof spray solution might yield reductions in the 25 - 30% range, but at much less favorable costs for implementation. Remember that the roof/attic is only part of the overall cooling load in a building. Windows are at least 20% of the total and walls, doors and infiltration at least a similar amount.
- Danny Parker
Richard Nelson wrote: Cool Roofs - a solution to the California Energy Crisis? I applaud the work you have done at the FSEC concerning the construction of roofs and the resultant air-conditioning loads. May I please offer these further comments and invite your feedback and discussion on this very important and neglected subject:
An opaque roof of any color will become dirty and the savings in energy will diminish. In any case the army has studied different color tent materials to provide bio-contamination shelters and found that there was modest savings (reduction of air-conditioning load) for lighter vs. darker fabrics. Also roofs have been sprayed and irrigated with water to reduce their temperature. This is much more effective. At Sola Roof we provide a transparent roof that is equipped with a plant leaf canopy over the rooftop or at the roof level. The plants are watered hydroponically and cool by transpiration (removing heat energy in the form of latent energy). Additionally the transparent architectural fabric (Sola Fabric) can be cooled with local cold water resources (ocean, lake and groundwater).
Our savings - due to cooling load reduction are an order of magnitude greater than the Florida Study results (but I do commend this work and hope that your results cause us all to think more about the simple nature of the dominant cooling load - hot roofs)
Please have a look at our Websites - and I look forward to your thoughts on my White Paper on the California Energy Crisis below.
Dramatic Green Energy solution can eliminate the California Energy Crisis by Richard Nelson
How does our "Blue Green Building" concept apply to the serious electrical supply shortfall developing in California (and to the West Coast in general on account of the drought in the Northwest)?
I believe our powerful new construction technologies for building sustainable communities will be central to winning a victory for both the people of California and for the environment. We need a win-win.
What I find very disturbing is the temptation to abandon the clean air regulations in the face of this crisis. If this happens the path is opened to simply increase the electricity supply by building many more thermal power plants. I believe that this battle must be won by achieving a substantial improvement in the energy efficiency of cooling and lighting buildings because this where the majority of the demand (I estimate at over 25,000 Mw peak) for electricity originates.
I am urging a Green Energy solution obtainable through the use of advanced Green Building technology that provides for economic growth free from the problems that are now restricting our individual and collective development. These natural limits have been reached because our lifestyle is out of balance with the natural environment. For this situation to change, our homes and communities should reflect a deeper harmony with nature.
I would like to share with you my insight in relation to the above that may provide some answers by means of a new and advanced construction technology, which I call the Sola Roof. This is a transparent roof construction method that, while it provides shelter and day lighting to buildings and ground level spaces beneath, also is equipped to grow an extensive plant leaf-canopy at the roof-level to capture and transform the sunlight for various uses.
The urban landscape would be transformed by this technology into vibrant, productive, cool and verdant garden-like spaces.
Perhaps you share my concern that the timing of these electric power blackouts occurring in California is very important. Though they may appear relatively minor and unimportant - these events can be early warnings of bigger problems ahead. I hope they can heighten our awareness to issues that have been long avoided. Let us face the problem squarely and find real solutions before larger crises overtake us. Solar energy and other alternative energy (electrical from wind as well) technologies have not yet delivered on their great potential. The current volume of production of these new products results in unit costs that are still too high and this situation seriously limits any expansion (installed capacity is less than 1% of the energy demand). We have at present no viable, large-scale (manmade) solar energy conversion systems that are cost effective.
But wait! What if we adopt the massively successful ecological mechanisms of the natural world? Though they are not manmade - plants are responsible for the proper function of the world ecology. Why not use plant based technology to provide the solar processes that we need?
Photosynthesis (depending on the crop and the level of CO 2 enrichment) can reach efficiencies of 3 to 30% in the conversion of the solar radiation to hydrocarbon. In this process CO 2 is sequestered in the Biomass that is produced. PV and other solar devices do not create hydrocarbons (which we know and understand so well). A leaf canopy can be grown hydroponically (low cost mass production) within the Sola Roof superstructure, which can span over the vast areas of unused roofs throughout our cities with many benefits that include efficient solar energy conversion and load reduction for the cooling of buildings located beneath the leaf canopy. These are the truly "Green Communities and Buildings" of the future.
Look at Phytotechnology this way: sunlight is clean energy for plant growth but CO 2 is the input that most limits their efficiency in the production of hydrocarbon. In a closed atmosphere controlled environment we can use elevated levels of CO 2 in the range from 1000 to 3000 PPM (several times the normal atmospheric levels). Not only food and horticulture crops but also algae can be produced (in a CO 2 enriched atmosphere) at greatly enhanced rates of growth compared to field conditions.
I believe that the 21st century can be the beginning of a Green Millennium that will rely upon plants for the production of biomass and bacteria for the conversion of this hydrocarbon to methane (and nitrogen rich nutrients for plant root-zone fertilizer). The methane so produced can be processed by steam reformation into hydrogen rich gas that is then used to feed fuel cell reactors that will chemically produce electrical energy. The CO 2 that would be emitted by the fuel cell can be delivered to a CO 2 enriched atmosphere plant production system (like our Sola Roof). New (high-pressure) clean coal combustion technologies or gasification/fuel cell systems will produce both clean electric power and liquid CO 2 that is a resource to be bottled and shipped to the Phytotechnology crop production systems.
The Sola Roof is a new style of "Green Building" that would produce food and horticultural products as well as environmental benefits. Also, the solar energy received through the transparent roof-cover is transformed by the plants into useful energy products; including cold water & hot water, comfortable and productive living, working and recreational space, pure water (from condensation), rainwater collection and Biomass production for Bio Fuels?, Bio Chemicals? and Bio Pharmaceuticals?.
Of course the Solaroof technology is specifically designed for this water cooling process and provides at the same time natural daylight within the structure. Additionally, roof areas with our integrated hydroponic crop production system -- the Sola Roof -- can produce a very exceptional income in addition to the savings on air-conditioning costs. One must understand the vast potential of tapping into the infinite cooling capacity of the Pacific Ocean.
By comparison the energy cost of simply pumping cold ocean water to building developments built in proximity to the coastline is 50 times more efficient than the electricity/air-conditioning scenario described above. We must remember that 80% of the populations live on the coastal plains. Using the cooling water resource directly simply requires building envelopes (especially roofs) better adapted to liquid cooling. Our Sola Roof technology is designed with this capability and can certainly lead the way. Not only does this roof system not overheat -- but additionally it provides cool, modulated daylight under a leaf canopy and transparent membrane -- which is stripped of the heat producing infrared radiation and the damaging ultraviolet radiation.
This concept will blow the lid off the constraints on economic growth in California. Can you picture the extent of the Green Communities and sustainable building developments that could occur inland of the coast between LA and San Diego? Instead of massive new thermal power plants along the coastline there would be much smaller electric powered (no air pollution) ocean water pumping stations (district cooling) to supply the air-conditioning needs of these communities. Publicly financed residential developments could draw away development pressures from older, in inefficient city centers. Additionally, our Sola Roof structure systems are extremely resistant to earthquake and the structures pose no collapse risk whatever. I believe we only need a couple of well-placed demonstration projects to trigger a massive interest in our products this summer.
1) Highly reflective white roofs have both high reflectivity and high emissivity. (White paint has a tested reflectivity of up to 80% with a surface long-wave emissivity of 0.88 - 0.94). So, you need to correct your understanding on that issue. Note that galvanized un painted metal roofs have a fairly high reflectance, but very low emissivity. They do get very hot (180 F +). They are not a good choice for a tropical environment (Tennessee Williams "cat on a hot tin roof.") . Painted white is quite different. They reach temperatures which are only slightly above ambient outdoor temperatures and fall quite below ambient temperature during the evening hours (beneficial cooling from the night sky radiation).
2) Reflectance and emissivity properties of roofing materials: http://www.fsec.ucf.edu/~bdac/pubs/CR670/CR670.html See Table 9 for unpainted aluminum and galvanized roofing vs. Table 3 for white metal.
3) We have taken very rigorous data on roof thermal performance over the last ten years in side-by-side highly instrumented roofing test cells: http://www.fsec.ucf.edu/~bdac/pubs/PF337/FRFPPR.html
4) Roof temperatures: You are familiar with the data we have on the homes from Ft. Myers: http://www.fsec.ucf.edu/%7Ebdac/pubs/coolroof/exsum.htm See Figure E-3 that shows the attic temperatures over the monitoring period. I attach a figure showing the corresponding measured average roof surface temperature over the same timeframe. The average maximum air temperature reached about 89 F. These are averages for a three week period in summer.
Also, I attach data for an extreme day, July 26th-- one of the hottest we had during the monitoring period. That gives you an idea of how reflectivity governs roof surface temperature.
5) You seem to have doubts about our "lack of empirical data." We measured these temperatures. We measured the 17 - 23% drop in AC power from reflective roofing and nearly 33% on peak. Given that ALL that I show you is based on detailed, scientific measurements (temperatures, air condition power etc), what would comprise sufficient empirical data for you? Contrary to what you indicate, there is no "exaggeration" in our evaluation. We pride ourselves on being objective in our assessments and let the chips fall where they may. The way the chips have fallen show that surface reflectivity is one of the most important properties governing the thermal performance of buildings in cooling climates.
6) I would expect thatched roofs to do quite well. The thatched grass likely has a reflectivity in the 40% range and they have a high emissivity. The thatch promotes convective abatement of the heat being collected on the roof and the thatch also functions as insulation. That said, I think you could conclusively show with test cell data that a white metal roof would considerably outperform thatch because it would cool so readily to the night sky and reduce temperatures rapidly during evening hours. Insulation is a disadvantage at night with cool night skies.
7) We have done detailed work to evaluate the "heating penalty" of reflective roofing and how that compares with the cooling season advantages:
D.S. Parker, Y.J. Huang, S.J. Konopacki, L.M. Gartland, J.R. Sherwin and L. Gu, "Measured and Simulated Performance of Reflective Roofing in Residential Buildings," ASHRAE Transactions, 1998.
This is a per reviewed paper (American Society of Heating, Refrigerating and Air Conditioning Engineers) showing a detailed simulation analysis is locations around the U.S. Bottom line: reflective roofing is an advantage below 40 degrees north latitude.
8) I would expect evaporative roof cooling methods to yield large savings. I would not deny that. There are many clever schemes (see Baruch Givoni: _Passive and Low Energy Cooling of Buildings, Van Nostrand Reinhold, 1994). However, I remain unconvinced that these methods are low in cost. Having reviewed your slides illustrating the concepts involved, while interesting, I would say they fall under that heading: complex, mechanically and control intensive and likely expensive.
- Danny Parker
Dear Danny Parker,
http://www.energy.ca.gov/commission/2001-02-22_rosenfeld_wsj.html challenge to some of your conclusions.
We have found some common ground of understanding above, but I would like to continue to challenge both you and your organization as to some of your other conclusions (or assumptions). I would start by saying that constructing buildings with white roofs in the south is something that you have proven to be important but could not be described as a modern innovation. In fact it is a forgotten wisdom that is now being relearned and rigorously proven with modern data acquisition and computer modeling and analysis tools. What about looking at some really new innovations in solar structure design? I would appreciate the opportunity to collaborate in this area.
Preface to my Question: What about the effect of water films on or in roof cavity spaces?
My first advanced proposal sets the goal of achieving better than ventilated building comfort levels with one tenth the energy typically used for ventilation by conventional practice and state of the art know-how. This result can be accomplished by growing a rooftop plant leaf canopy (root mist hydroponic system) on a horizontal net over a translucent fabric roof; this version is called Sola Roof Garden.
My second advanced proposal is the Phyto Technology? system where the Liquid Solar Technology is used in a roof cavity space between a two layer transparent fabric roof (together with district water cooling) to provide better than air-conditioned comfort levels at one tenth of the energy consumption of your best comparison (most efficient state of the art building design for air-conditioned spaces). This system uses a plant leaf canopy just beneath the Sola Roof transparent roof construction system. I expect this energy efficiency to be attained without reference to the benefit of day lighting or the conversion of solar radiation to biomass.
The third degree of advancement is the use of the Sola Roof evaporative chilling process, which requires that more energy be consumed than in the first two strategies, above and at the same time water is also consumed. It should be considered as part of a roof integrated solar thermal and PV to electricity technology, which is a longer-range project needing considerable development input.
All we need to do to have all the equivalent benefit as air-conditioning is to expend 50 times LESS energy and just pump the cooling water to the buildings. The pumping requirements are actually less than with the electrical/mechanical approach because the reduced demand will save the construction (or operation) of the thermal power plants. Each avoided kilowatt saves two kilowatts of thermal waste heat. How does the power plant discharge this waste heat? Answer: By pumping two times the volume of cooling water as would be needed to absorb the cooling load directly. The power plants pump tens of thousands of cubic yards of water per day. It is considered as small energy penalty for creating electricity. But, how foolish is it to use that precious electricity to produce cooling in buildings when cold water can do the job.
Questions on the third stage study:
Can Thin Film PV be deposited on a thin transparent architectural fabric?
What spectrum of the solar radiation would pass through the architectural PV/Fabric laminate?
We use the Liquid Solar film as a shutter to let it into the building for passive warming or block and absorb to create cool daylight. The visible is only slightly absorbed by a thin water-based Liquid Solar film. The Liquid Solar film used to capture energy (a water reservoir thermal mass is used for diurnal heat storage and release) or for flow through heat rejection (district cooling). The cooling water flow rate can be further reduced for heat rejection by using an evaporative chilling process in the roof cavity of the Sola Roof.
This approach provides a best use of the entire radiative spectrum:
Could you introduce me to your PV associates to further discuss the great potential of this integrated system approach? I would love to collaborate to create this architectural PV Sola Fabric.