November 10th, 2008
This paper summarizes the work that has been funded by the Sacramento Municipal Utility District (SMUD). The issues that were identified as a result of this work and as a result of on-going contact with the white coatings industry are discussed. The paper outlines on-going and future work that SMUD intends to perform to address the issues that have been identified.
The emphasis in this paper is on white coatings because much of the white roof activity in Sacramento has involved this class of products. SMUD is also interested in other types of highly-reflective roofing products. The issues discussed below apply, for the most part, to all reflective roofing products.
The projects discussed in this paper were funded by SMUD, the electric utility for Sacramento, California. Much of the work was performed by Lawrence Berkeley Laboratory (LBL< ). SMUD serves about 450,000 customers and has a summer peak load of about 2,100 MW.
Sacramento experiences summer peak temperatures of 110°F or more, making SMUD a significantly summer-peaking utility. This is in spite of the fact that, in terms of degree days, Sacramento’s heating season is almost 250% larger than the cooling season. Occasional extreme heat in the summer and moderate temperatures in the winter give SMUD a poor load factor. This situation is aggravated by a climatic condition called “the Delta breezes”. The city of Sacramento sits in the middle of California’s huge Central Valley, which is separated from the Pacific Coast by ranges of low coastal mountains, which are broken only by the Sacramento/San Joaquin Delta. On hot summer days, hot air in the Sacramento Valley rises and cool ocean air is drawn into the valley, arriving in time to assure nighttime low temperatures ranging from roughly 50°F to 75°F with temperatures most typically occuring in the mid-50’s to mid-60’s
Many homeowners in Sacramento use whole house fans, window fans, or simply open all of their windows to cool off their houses at night. During the day, they close up their houses and use their air-conditioners only during the most extreme heat. A segment of SMUD’s residential rate payers pride themselves on almost never using their air conditioners. Nevertheless, under heat-storm conditions, when peak temperatures exceed 108°F for two consecutive days or more, most of Sacramento’s air conditioners operate across SMUD’s peak period.
Thus, the SMUD system must accomodate massive loads imposed by air conditioning for relatively short periods of time. SMUD has three alternatives: it can reduce summer peak loads; build new generating facilities; or purchase electricity from utilities and other agencies with excess generating capacity. SMUD has chosen to pursue all three options. White reflective roofs are bring studied as one of a number of demand-side technologies capable of reducing summer peak loads.
In 1991, SMUD installed white roofs on a home and two school buildings. SMUD and Lawrence Berkeley Laboratory (LBL) monitored the installations and LBL analyzed the data, which is summarized in an LBL report, “Monitoring Peak Power And Cooling Energy Savings of Shade Trees and White Surfaces in the SMUD Service Area: Data Analysis, Simulations and Results”.
The home was 29 years old, single-story, and had a cathedral ceiling with R-11 insulation. The air distribution system was located beneath the floors. The house had single pane windows and R-8 insulation in the walls. It was surrounded by mature trees which shaded the walls and small portions of the roof. The roof was coated with a locally produced acrylic/polymeric white coating. Pre-installation albedo was 18%. This increased to 79% after the coating was installed. Cooling energy savings were 80%, and peak demand reduction was 17%. The roof was revisited in the spring of 1992. A layer of dirt had reduced the albedo somewhat, but a thorough washing brought the albedo up to 73%.
The school buildings were small, “temporary” bungalows, each containing one classroom. The roof, west-facing walls, and south-facing walls were coated with the same product used on the home. The ceiling had R-19 insulation, the walls had R-11 insulation and the windows were dual-pane. Energy savings averaged 34% and peak demand savings averaged 32% for the two buildings.
In 1993, LBL examined a series of roofs for three specific products. Two of the products were on installations up to six years old. The third product was newer and its oldest application had an age of two years. All three products showed acceptable resistance to dirt and weathering. One of the products exhibited excellent cleanability: at one site its albedo returned to its original value after cleaning. Another of the products showed virtually no accumulation of dirt or deterioration in albedo after six years. Due to the small sample size, these findings are regarded as preliminary. Nevertheless, they do suggest that there are products capable of providing good albedo for a minimum of six years and probably considerably longer.
Wooden shake roofs (not a good idea in fire-prone areas) are popular in Sacramento. These roofs are relatively expensive to maintain, typically requiring work at an age of 10-20 years and replacement in 15-30 years. In 1992, SMUD installed a white roof on a home with a 26-year old shake roof to test the feasibility of using a white coating to defer or eliminate the need for more expensive maintenance. The coating is performing well.
In implementing white-roof products, SMUD’s goals are to determine: (1) the costs and benefits of white roofs; (2) decide whether or not to embark on a white-roof program and (3) the possible components of the program. To accomplish each of these goals, it is necessary to know the long-term benefits of white roofs. Recent projects have provided information on short-term performance but more work is needed. Virtually no work has been done on long-term performance. The issues discussed below tend to center on the need for long-term data. Among the issues, “albedo maintenance” is perhaps the most serious issue.
Completed projects show significant savings for white roofs, but the number of projects is limited and the database is very small. A small database makes it difficult and perhaps impossible to quantify demand and energy savings for a large population of homes or buildings with white roofs. Using residences as an example and ignoring issues such as variations in product performance and the effects of dirt and weathering, there are large variations in energy-use characteristics, even among houses that appear to be similar. There are several reasons for this: (1) variations in the characteristics of homes; (2) differing locations and performance characteristics of air distributions systems; and (3) varying habits and preferences of homeowners and renters.
To address this issue, utilities, the coatings industry and other interested agencies should develop a relatively large body of case studies, backed by long-term monitoring and accurate data analysis.
Albedo maintenance refers to the ability of a reflective roof to maintain a high albedo over a long period of time. Albedo maintenance is emerging as perhaps the most serious issue regarding white roofs, driven by a large difference between short-term and long-term marginal costs for electrical demand and energy.
At SMUD, short-term marginal costs for demand and energy are relatively small but become much larger over the long-term. As a result, long-term measures are the most beneficial. There is little data on the effects of weathering and the accumulation of dirt on the performance of white roofs. Research which proves the existence of substantial long-term benefits would likely create greater enthusiasm for white roofs among summer-peaking utilities.
Weathering, which may slowly reduce the thickeness of roof coating, and the accumulation of dirt over time may reduce the albedo of a white roof. If this occurs, cooling loads will increase as albedo decreases. This is an important issue because there is little long-term data on albedo degradation. If it is assumed that weathering and dirt substantially degrade the performance of white roofs, cleanability becomes an issue. That is, can albedo be restored through cleaning? If so, can this be accomplished in a cost-effective manner and what percentage of the original albedo can be restored?
In general, it is not known how long a given coating will last in any given situation. In addition to those considerations given above, other factor also appear to be important. These include: (1) the thickness of the coating; (2) the chemical composition of the coating and adherence to the manufacturer’s guidelines for mixing and additives; (3) the quality of the application; (4) the type of substrate to which the coating is applied; and (5) the effects of ponding water.
At SMUD, we intend to address these issues by measuring the albedo of white roofs in the field over a long period of time. There a huge number of white coatings and other highly-reflective roofing products and a nation-wide effort would be extremely helpful.
The issues discussed above point to the need for industry-standard testing of white roof coatings and a corresponding labeling program. Testing should include long-term monitoring of the effects of dirt and weathering, augmented by accelerated life-cycle testing in the laboratory.
The existing computer programs, such as DOE-2, seem to under-predict the benefits of white roofs in California. I perceive at least three reasons for this: (1) the algorithms contain simplifying assumptions which may not be valid for hot, dry climates; (2) effects on air-distribution sytems are not considered; and (3) radiant effects on comfort are not included.
Some manufacturers now provide coatings which contain ceramics and other additives. The benefits of these additives should be determined using laboratory testing and long-term in-situ testing.
The results of work performed by SMUD and Lawrence Berkeley Laboratory (LBL) show that white roofs may be able to significantly reduce summer peak air conditioning loads. (A considerable amount of work has been performed by the Florida Solar Energy Center, with promising results.)
Based on these results, SMUD has decided to embark on an expanded program which will implement white roofs and monitor their performance. Projects will be completed in the residential, multi-family, small commercial and large commercial sectors. SMUD will co-fund, with building owners, the projects listed below. Co-funding, as opposed to full funding by SMUD, is expected to be adequate because there are a number of white, high-albedo roofing products with good track records.
In the residential sector, 30 or more homes will receive white roofs. In addition to creating a larger data base, it is hoped that these projects will reduce aesthetics as a barrier to the more wide-spread use of white roofs in the residential sector. Monitoring will be simplified compared to SMUD’s previous white roof projects. It will consist of attic temperatures and cooling energy consumption. Site will be visited in future years to determine albedo maintenance, cleanability and the overall durability of the white roof.
In the multifamily sector, SMUD is providing a large rebate so that a white coating will be used on all 61 buildings in a local low-income housing project. Large numbers of shade trees may also be planted as part of the project. This is presently regarded as a long term project: Installation of white roofs will not be completed until 1995 and installation of air-conditioning will not be completed for several years. The project should be monitored for attic temperatures, space temperatures and energy use by air conditioning units. Ambient air temperatures throughout the project will be monitored in an attempt to detect a reduction in the urban heat island effect.
In the small commercial sector, SMUD will fund three projects. Monitoring will consist of air conditioning energy use and air temperatures beneath the roof. SMUD will also co-fund two large commercial projects. The monitoring will be similar to that of the small commercial projects.
With the exception of the low-income housing project, all of the monitoring work discussed above addresses direct effects of white roofs. Indirect effects, which is defined as the reduction of ambient air temperature, are also expected to be important if white roofs, light-colored paving and/or shade trees are widely implemented in a neighborhood. Additional work on indirect effects is needed.
Aesthetics is a significant barrier to the use of white roofs in Sacramento. SMUD will track work on pigmented or lightly-pigmented coatings and other roofing products that maintain high albedo. Promising products will be used in demonstration projects.
There is a considerable debate about the benefits of ceramics and other additives in white coatings. SMUD intends to submit coating samples for laboratory testing.
In summary, there is a great need for a nation-wide testing and certification program for roof coatings. There is a need for monitoring and case studies by independent agencies such as utilities, state energy offices and the federal government. Existing computer models require improvements and should be validated using data obtained by monitoring actual installations.
Tags: Acrylic Coatings, Air Conditioning, Albedo, Lawrence Berkeley National Laboratory, Ponding Water, Reflective Roofing, Roof Coatings, White Roof
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