Designing an Integrated Low-Cost Solar Collection System

Introduction

Exploitation of non-renewable resources (e.g. materials, energy, etc.) can create non-sustainable conditions and environmental degradation, resulting in a significant loss of final product quality.  In many industrialized countries, including the United States, the heating, cooling, ventilation, and lighting of buildings represent approximately 40% of the nation’s annual energy consumption (Hartkopf et. al 1994). Therefore, it is important to seek alternative methodologies that continue to improve our quality of life, while reducing energy and environmental consumption.

In the past 20 years, interest has grown in developing new methodologies that can effectively utilize renewable sources of energy, an effort that has yielded a variety of techniques with differing degrees of promise.  Recently, researchers have strenuously promoted the use of solar energy as a promising alternative  (Ametek 1985). Solar energy possesses characteristics that make it highly attractive as a primary energy source that can be integrated into local and regional power supplies; essentially, it represents a sustainable, environmentally friendly source of energy that can reduce energy costs (Lundsager 1996).  Although recent studies have shown that the use of solar energy is strongly supported by the US public, its up-front financial expenditures are greater than those associated with traditional systems, which makes some critics doubt its overall cost-effectiveness (Rockwell 1999).  In fact, this doubt has led to a certain amount of mistrust among agencies as to the true long-term benefits of solar-based systems.

In light of the aforementioned discussion, the objective of this research is to present a new, more cost-effective methodology for integrating solar energy collection systems into conventional building structures.  This goal involved rethinking the overall building construction methodology to balance the increase in cost associated with the solar systems by a decrease in other construction costs.  In the future, the potential exists for building an instrument to experimentally test the proposed final design.

Background

System Integration for Building Performance

The redesign process relied on integration theory, a process that develops properly integrated designs by dividing the building into four systems that address its major functions: structure, interior, envelope and mechanical.  In any buildings, the aforementioned systems are interconnected, and the nature of the connection identifies the level of integration.  The designer can then investigate alternative levels of integration to conserve space, material, and time (Rush 1986). 

The level of integration can, however, be constrained by the accepted level of expected performance.  Integration theory identifies six main performance categories: spatial, thermal, air quality, acoustical, visual, and building integrity.  Each performance category defines the “comfort zone” accepted by the building’s occupants.  It should be noted that decisions made within each system—structure, envelope, mechanical or interior—affects different performance categories depending upon the level of integration. 

The aforementioned six performance criteria are integrative and should be maintained through proper communication during the building’s delivery process, from the design stage to the development of the integrated system.  The result will be a building that performs in harmony over time and fulfills any client’s central requirements: suitability, reliability, and flexibility (Rush 1986).

Solar Energy Collection System Integration

Properly integrating a solar energy collection system into a building’s shell and mechanical systems could reduce the cost of the system as well improve collection efficiency. Therefore, research in Building Integrated Solar Thermal (BIST) design, which started in the early 1940s, continues unabated. In solar integrated design, the building is used as part of the collection system.  BIST systems have many advantages over normal collection systems, including their ability of expanding to cover the entire area of the roof at a reasonable cost.  Additionally, these systems usually have a longer service life, which results in less of a need to replace materials, as well as in improved conservation.

 Solar integrated designs face four major concerns that affect client acceptance: thermal performance, building envelope performance, aesthetics, and cost-effectiveness.  Although many of the proposed designs failed in one or more of these categories, some proved successful.  Unfortunately, even for the successful designs, the supporting research necessary to improve aesthetics and performance, as well as lower system costs lagged behind (Archibald 1999). 

In many successful designs, the roof was distinctively used as an integrative thermal building element, which means that it performed its normal functions—formation of a thermal and vapor barrier, resistance of wind and weight of snow and ice, providing of a comfortable environment for the space—but also collected energy regardless of design (i.e. active, glazed, unglazed, passive, focusing, or non-focusing).  Combining the function of two separate systems into one resulted in a cost reduction of up to sixty percent in materials, time, and labor.

Formulation of the Design Criteria Based on Building Integration

This research focused on integrating active solar collection systems with the building’s roofing system.  The primary goal was to integrate the envelope, structure, and mechanical elements in order to provide better building envelope and thermal performance and to improve aesthetics at a competitive cost.  The proposed roofing collection system was designed for a single location: Blacksburg, Virginia.

The following requirements were satisfied in the formulated design:         

1.        Thermal performance: to provide the building with heating and hot water, act as an energy storage sink, maintain an appropriate air temperature, mediate radiant temperature and relative humidity within the space, and maintain a comfortable level of air movement.

2.        Environmental design: to design a durable, flexible and sustainable building.

3.        Building envelope: to act as part of a weather tight envelope, prevent precipitation from entering the building, keep out wind, dust and odor, and control the temperature and humidity inside the building.

4.        Aesthetics: to blend the collection system into the building’s architectural makeup and introduce a marketable product.

5.        Cost-efficiency: to reduce construction costs, extend the system’s service life, and produce a competitive alternative to traditional systems.

To satisfy the aforementioned requirements and create this new solution for residential construction, a multidisciplinary research team of building constructors, engineers, and architects was formed.  This team was made up of graduate (Masters and PhD) and undergraduate students from mechanical, building construction, and civil engineering, and architecture. It started out as an academic exercise then became a research project, students volunteered to join based on their research interest. The project has been in operation the past year and a half.

Current Available Low Cost Flat Plate Solar Collection Systems

A variety of solar collection techniques exist, including photovoltaic ones that convert heat energy to electricity and flat plate ones that collect solar energy to provide heating and hot water requirements.  Flat plate collectors were chosen for usage in our research because of their ability to supply energy at low cost. In addition, they can be easily integrated within the structural members of the building.

The flat plate design represents the most economical, active method of solar energy collection.  Such designs consist of an assembly of transparent covers over an absorber plate backed with thermal insulation, as shown in Figure 1.  The cover prevents convection losses, reduces thermal radiation losses, and protects the absorber plate against environmental hazards.  While the absorber is a coated plate upon which the sun’s energy is converted to heat, the insulation prevents back losses. Flat plate collectors use absorbers to heat air or water, which can then be used to heat water or stored for later use. They are usually chosen for applications requiring moderate heat gain (i.e. 100^oC above ambient temperature; ASHRAE 1999).

Theoretically, when the sun is directly overhead on a cloudless day, a flat plate collector with 10m² of surface could provide energy at 10% efficiency of collection. That rate of efficiency decreases when convection and radiation losses occur (Goswami et. al. 2000). 

Design process

The proposed integrated solar collector roof panels were designed to accommodate the aforementioned design criteria. This section will examine available alternatives for achieving the optimum design in terms of thermal performance, environmental design, building envelope, aesthetics, and cost-effectiveness.

Thermal Performance

To accommodate the thermal performance requirements, the integrated roof should serve three functions: act as an efficient solar flat plate collector, utilize its thermal capacitance for storage, and retard the propagation of the thermal energy through the building component.  These criteria can be accomplished by integrating a flat plate collector with a high thermal inertia material, also known as a thermal mass.  Research has shown that there is great potential for storing thermal energy within the structure of buildings (Simmonds 1991).  Thermal mass can delay heat transfer through a building component, resulting in only moderate indoor temperature fluctuations during outdoor temperature swings. In addition, if properly sized for energy storage and in comparison to a similar low-mass building, thermal mass reduces the building’s energy consumption and shifts energy demand during off-peak periods (ASHRAE 1999).

Advantages that can be attained through thermal mass usage include: reduction of peak electrical demands; utilization of low nighttime electrical rates, if needed; offsetting mechanical cooling with free night cooling; and enhancing of equipment operation at more favorable part-load conditions (Braun 1990).  Therefore, it was decided that this project should utilize a building material with a high thermal inertia.  Table 1 summarizes the thermophysical properties of the different alternative materials this research could use (after Incropera and De Witt 1996).  As can be seen from Table 1, concrete has the highest thermal capacity, which indicates that it has the maximum heat storage capacity (1936KJ/m3K).  Although wood can provide a longer time lag, it is more expensive to manufacture in larger thicknesses.

Table 1

Thermo-Physical Properties of Building Materials (Incropera and De Witt 1996)

Environmental Design

The environmental requirement allows the proposed system to function in harmony with its surroundings.  The solar collector integrated roof was located on the building’s façade in a position that would allow it to capture the largest amount of solar energy.  In order to assure maximum solar exposure, it was also inclined at an equivalent angle to its location’s latitude. Since this research models a case study located in Blacksburg, Virginia, the integrated solar roof will be located on the south façade and +/- 30 degrees from south at an inclined angle of 37^o.

One of the key features of such a system is flexibility, the ability to achieve any desired shape, finish or geometry at any point in time to adapt to changing functions and occupancies.  The system created for this research project was designed for maximum flexibility.  In order to achieve this objective, the appropriate building system should be capable of taking many desired forms or geometries at a competitive cost.  Negative formwork was used to achieve this objective.  These forms are manufactured by spraying polyurethane foam on a material that gives the desired surface finish, such as stone, and color pigments can be added to achieve a natural tone.  When the polyurethane hardens, it is stripped off and used as the inner lining of the formwork for the concrete system.  The outer lining gives rigidity to the formwork and can be made out of wood or plastic.  Such formwork can be used to produce a minimum of 5000 panels, which makes the process attractive, cost effective, and easy to incorporate into an industrialized production line.

A system must also be reliable. Reliability is defined as the probability that, given proper maintenance, performance will continue as intended through the building’s life.  Current construction practices indicate that  the use of layered construction has been increasing for the past decade and, along with it, so has the need for maintenance, increasing specialization of trade, and increasing losses of time and money. Therefore, a goal of this research is to eliminate layered construction in order to minimize system failure and increase reliability.  Utilizing negative formwork with a concrete system allows for construction of single composite systems.  The polyurethane  and concrete layers are structurally joined using high strength, low conductive fiber composite connectors that are flexible to allow for different volumetric changes.  They are also strong enough to resist forces placed on the panel that could otherwise lead to stripping.  Such connectors come in various forms: a C tie, a hairpin tie, a No. 2 tie, a Stirrup tie,  or a plastic nail.  When embedded in concrete, even during freeze-thaw cycles, they experienced no loss in strength. A key advantage of such connection techniques involves elimination of the bridging effect that occurs when metal ties are used (Sauter 1991).

Finally, such a system must also be sustainable:  together with its integrated parts, it must serve the user’s needs in the present as well as in the future.  Therefore the system is required to be durable as well as resistant to natural disasters including floods, hurricanes, and fires.  Concrete structures can be designed for a service life of up to 100 years and can use available technology to accommodate the aforementioned requirements.

Building Envelope

Reinforced concrete building shells have a proven ability to keep out wind, dust, and odor.  If properly insulated, they can prevent precipitation from entering the building and help control the temperature and humidity, thereby satisfying the building envelope requirements.

Aesthetics

 The architecture team working on this project has developed a design that evolved over time from a 12 sided, three-story townhouse, as shown in Figure 2, to a structure that combines rectangular and six-sided shapes, as shown in Figure 3. Floor plans for the final design are shown in Figure 4.

Integrated Roof Solar Collector Description

Design of the integrated roof solar collector was shaped by an iterative process of design, performance evaluation and redesign. Two alternatives reached the final stages of design.  A cross-section of the first design is shown in Figure 5. It consists of a 6.35 mm (0.25 inch) single glass panel with an e-coating on the inner surface, followed by an air-gap.  The thermal collecting medium consists of a fluid (water with antifreeze) enclosed in 12.7 mm (0.5 inch) diameter copper pipes painted black to ensure maximum solar absorption. 

The pipes are laid within concrete cavities to minimize construction cost and time, as well as to reduce convection losses.  They are surrounded with aluminum foil to ensure reflection of any radiation hitting the inner surfaces of the concrete cavity back to hit the copper pipes.  The fluid absorbs the solar energy and transports it to storage locations.  The copper pipes are connected by the aluminum foil layer surrounding them.  The parts of the aluminum foil not surrounding the pipes are also painted black to ensure maximization of the absorbed radiation.  The foil absorbs the solar radiation and transmits it to the pipes by conduction (Duffie and Beckman 1980).  The copper pipes are placed in groups of three every 800 mm (31.5 inch), a distance selected to maximize temperature gain while minimizing panel cost.

A 50.8-mm (2.0 inch) layer of insulation is attached to the back of the aluminum foil to reduce heat transfer to the adjacent 76.2 mm (3.0 inch) concrete.  A 76.2-mm (3.0 inch) concrete layer is used to prevent growth of microbes in the polyurethane layer and thus ensure healthy air quality within the system.  Polyurethane foam 76.2-mm (3.0 inches) thick is placed after the concrete to act as an insulation layer.  The last layer consists of concrete 76.2 mm (3.0 inches) in thickness to ensure structural stability as well as contribute to the thermal inertia on the inside of the facility.  The panel is connected by the aformentioned fiber-reinforced plastic stirrups, commonly used in slab construction, to provide tensile and shear strength. Such stirrups help the panel function as a composite system, rather than a multi-layer one, and resist temperature and loading changes (Sauter 1991). 

To quantify the benefits of the first new design, finite element models were developed to predict the integrated solar collector’s thermal performance and compare it to that of traditional flat plate collector designs.  ABAQUS Software, Version 5.8 was used for the finite element modeling of the solar roof panels (ABAQUS 1998). The developed finite element code represents the solar panel shown in Figure 1, with a length and width of a typically inclined two or three-story building roof. The roof’s incline is an input parameter (modeled at 37^o to maximize solar gain). To investigate the solar panel’s effectiveness, three dimensional models were developed. The dimensions of the modeled portion are, in each case, 3500mm x 1500mm (138.0 inch x 59.0 inch). These dimensions were selected to reduce any edge effect errors while keeping the element size within acceptable limits (modeling constraints). The roof panel geometry dictated the element types and dimensions. Rectangular (DC3D8-brick) continuum elements (with a “brick” defined as an 8-noded element) were selected for all materials in the 3D model, except for the fluids, where rectangular (DCC3D8-brick) continuum elements were used to simulate the forced convection taking place.

Solar intensity and inside and outside temperatures are assumed to be user-defined, taken from weather data in TMY2 files. A convection boundary condition and a solar radiant flux govern the heat exchange taking place between the environment and the glass surface. The convection coefficient (h_c) is defined as follows (Duffie and Beckman 1980):

h_c = max [5, 8.6(V0.6/L0.4)]  (1)

Where, v is the wind speed in m/s, and;

L is the cube root of the house’s volume.

As it passes through the glass, the radiative flux is reduced according to its angle of incidence. It passes through a subroutine that calculates the absorption, reflection, and transmittance according to the properties of the glass.

In comparison to that of traditional flat panels, results showed superior performance of the new design (Hassan and Beliveau 2003).  Other benefits include a significant reduction in construction costs: while the traditional flat plate solar collector costs $24 per square foot, the proposed integrated roof collector is a mere $15. In addition, use of a composite single system requires the presence of fewer trade representatives on site, thereby reducing labors costs. Additional savings can be achieved because the single-layer system requires less maintenance. Finally, another source of savings may be the use of precasting techniques and rapid in-place assembly, which in combination also reduce construction time and labor.

Unfortunately, early evaluations of the aforementioned design showed that it is difficult to construct and requires skilled labor. Therefore, it was simplified further, culminating in the design shown in Figure 6. As can be seen in this figure, the upper layer of concrete is covered with a layer of insulation and the circular concrete cavities are replaced with a 610 mm (2 ft) square cavity containing four 12.7 mm (0.5 inch) connected pipes.  This alteration further simplifies construction. To reduce the wind speed that results in convection coefficient and losses, the concrete has 76.2mm (3 inch) semi- cylindrical wind breakers placed every 610 mm (2 ft). The section shown in Figure 6 is repeated along the roof as needed to satisfy the building’s heating and hot water requirements.  The rest of the features remain the same as in the first design. With this revised model, cost and results improved. The cost of constructing the second design is around $10per square foot. Finite element analysis models also indicated that, when compared to that of traditional flat plate collectors, performance of the second model was superior.

Summary

A design process was presented for utilizing construction materials and techniques in the proper integration of low cost solar collection systems with the roof elements of residential buildings.  Such integration can enhance the performance of the collection system as well as reduce its costs.  The necessity for proper integration justified the need to evaluate the building design process. This paper summarizes the design process and presents the final design alternatives for the building integrated solar thermal roofing system. There is a possibility that construction of the designed system will begin in 2003. In that case, the building will be instrumented to evaluate and validate the design’s effectiveness and long-term benefits.

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