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By David R. Llewellyn, Senior Engineer in Process Controls Group, TJ Cross Engineers
Berry Petroleum (www.bry.com) is an independent energy company engaged in the production, development, acquisition, exploitation and exploration of crude oil and natural gas. Berry needed a system to produce steam for injection into an existing oil field reservoir. The steam was to be used to heat otherwise inaccessible heavy crude oil, reducing its viscosity and making extraction viable. This process is known as steam-based enhanced oil recovery or thermal EOR, and it's used in conventional and shale oil fields.
Berry wanted a system that would be inexpensive to purchase and install, simple to maintain and cheap to operate. They evaluated existing solar thermal systems and found them wanting, so they turned to our company, TJ Cross Engineers (www.tjcross.com), and to GlassPoint Solar (www.glasspoint.com) for a better solution.
The beauty of the GlassPoint solar water heating system lies in how its simplicity translates into greater efficiency, less maintenance, and lower installed and operating costs (Table 1). But creating a simple system can often be quite challenging, particularly when it comes to automation. Systems from AutomationDirect (www.automationdirect.com), enabled us to provide the required automation at low cost using a straightforward design.
Most thermal EOR systems worldwide burn fuel, primarily natural gas, to heat water and produce steam for injection into the oil reservoir. By contrast, GlassPoint's solar steam generators use the sun's radiant heat to preheat water to about 190 degrees F. This preheated water is then introduced to the natural gas-fired steam generator, greatly reducing fuel use and operating costs.
Solar energy systems of all types often employ very complex designs and expensive components. Solar energy systems that convert sunlight to thermal energy often require a host of complex subsystems to perform tasks such as precise alignment of mirrors, maintenance and cleaning of the mirrors, and actual conversion of sunlight to thermal energy.
For example, a typical solar thermal system uses parabolic mirrors to concentrate sunlight to one or more thermal conversion components located at focal points. These components vaporize water into steam, and this steam is used to drive a turbine connected to a generator. The entire system is extremely complex as it combines a steam-cycle power plant with a large scale solar mirror system and associated thermal conversion components.
By contrast, GlassPoint's solar water heating design employs commercial-grade greenhouses to house the entire solar concentrator mirror and water heating system. For this project, the single greenhouse structure is of galvanized steel and aluminum construction, with tempered 4mm glass roof and walls covering approximately 7,000 square feet of land.
The mirrors are made of an anodized aluminum reflective material used widely in commercial lighting fixtures and are durable for decades indoors, but not usable outdoors. The mirrors are light enough to be easily supported by the glasshouse structure.
All solar thermal systems require precise mirror positioning systems to optimize the efficiency of sunlight reflected from the mirrors to the thermal conversion components. Without a greenhouse, the mirrors must be quite rugged and heavy. Automated mirror cleaning systems are often needed, adding weight and complexity.
Because exposed mirrors and associated cleaning systems are quite heavy, the positioning systems must produce lots of torque. This adds up front costs, and also increases operating costs as relatively large motors are frequently started and stopped to reposition the mirrors.
By contrast, the mirrors in the GlassPoint system are very lightweight. This makes them less expensive to purchase, install and position. For this particular project, the mirrors are positioned using only the torque generated by eight 40-watt stepper motors and drives.
Perhaps the greatest advantage of enclosing the mirrors and the thermal conversion system in greenhouses is the increase in solar efficiency due to two factors: greater heat inside the greenhouse, and a more direct path from the sun to the mirrors.
It's hotter inside a greenhouse than outside, and the temperature differential can be as much as 30 degrees F on very sunny day. This higher temperature differential increases thermal conversion efficiency and boosts hot water output—ultimately lowering operating costs by cutting the use of natural gas in the steam generator. In a fortunate confluence of factors, many of the oil reservoirs that can benefit from steam injection are located in very sunny climes, further increasing benefits.
The thermal conversion device in the GlassPoint system is simplicity itself—just a carbon steel pipe. This pipe, or receiver tube, is located at the mirror focal point. The reflected sunlight illuminates the receiver pipe with 85 times more intensity than direct sunlight. The glasshouse structure provides a zero-wind environment inside, shielding the receiver pipe from wind-driven convective and conductive losses, and eliminating the need for an expensive glass envelope around the pipe.
This pipe runs from one end of the greenhouse to the other (Figure 2). Treated water at ambient temperature enters the pipe, is heated by the concentrated rays of the sun, and exits at a temperature of 190 degrees F for delivery to the steam generators.