Solar Thermal System Design

The Art and the Science

One of the happiest days at any solar thermal integration company is when the contract for a large commercial project is signed. The staff gets word that after months of hard work, the sales team has sealed the deal. Installers know they will be kept busy on a fresh project. Marketing begins thinking of ways to promote the system. Accounting prepares the schedule of values and other billing documents. Having fulfilled its role in the process, the sales team hands off the project to the design team, whose job now is to turn promises and projections into reality. In our last article (June/July 2011, SolarPro), we focused on system commissioning, the final step in the installation process. In this article, we turn our attention to an initial stage of the process: going from signed contract to final system design.

When the signed contract hits the designer’s desk, the clock starts ticking. It is time to dig in, examine the initial concepts for the system and produce a robust, polished design that will ultimately underpin a successful installation. Good designs lead to happy clients and stable systems that function and perform as promised. Bad designs lead to poorly functioning systems that cast doubt on the efficacy of solar thermal technology. More than any other part of the solar construction process, design is critical to the success of not only the solar thermal company, but also of the solar thermal industry itself.

Design Process Overview

There is no single correct or best way to conduct the design process. How a design unfolds depends on what the objectives are, such as aesthetics versus production or achieving a certain percentage of total usage with solar. Whom the designer is designing for—an experienced in-house installation crew or perhaps a plumbing contractor unfamiliar with solar thermal technology—is another factor that influences the process. This article is based on our experience in designing commercial solar thermal systems. Other solar integrators, engineering and architecture firms, HVAC companies and solar equipment manufacturers have their own design processes that can be just as effective.

The designer’s goal is to design a system that will last, that meets the client’s business needs and that generates profit for the solar thermal company. Clients expect a safe, reliable system that will last decades and help, rather than hinder, their business. A good design is the foundation of a solid system that is accepted by those who use it and that allows the system owner or stakeholders to shine as the individual or individuals who made the wise decision to invest in solar thermal. Designers must at all costs prevent the perception among clients and the general public that investing in solar thermal is not a sound decision.

Solar thermal system design is a complex calculus that factors in many variables, including:

  • Facility hot water load
  • Integration with existing water-heating or mechanical equipment
  • Collector type, size, angle, racking and mounting
  • Heat exchange
  • Flow rates, temperatures and pressures
  • Storage capacity
  • Pipe and pump sizes
  • Fluid and pipe expansion
  • Solar radiation
  • Equipment layout
  • System location and altitude
  • Aesthetics

These factors must be balanced with what can be done for what cost. Design is the “special sauce,” the alchemy that underpins all successful implementations of solar thermal technology. There are many installers and subcontractors who can build systems; unfortunately, there are currently far too few designers who can generate high-quality designs. Ideally, significantly more mechanical engineers will become competent solar thermal designers. Increasing the number of professionals who have mastered the art and science of solar thermal design is vital to achieving mass implementation of the technology and avoiding any black eyes to the industry, such as those suffered during the 1980s in the US.


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