Closed Loop Energy Modeling
Energy modeling is primarily used to evaluate the impacts of building energy efficiency strategies. Impacts are the differences in annual utility costs, size of HVAC equipment, carbon footprint, etc., of using a particular design or technology vs. conforming to a code-minimum or standard practice. Energy modeling is employed to calculate these impacts by creating two building models:
The proposed model: Intended to accurately reflect the energy performance of the building as-designed or built, which typically includes energy efficient technologies or design practices.
The baseline model: Employing the same geometric, operational and environmental characteristics as in the proposed model, but with properties of the energy efficiency technologies or design revised to represent the code-minimum or standard practice level.
By comparing the two models, energy analysts can determine the impacts of each energy efficiency measure (i.e., more insulation, high performance glazing, heat recovery) or for the complete design (all measures combined). This process usually occurs in the design phase as a method of evaluating the relative merits of different performance strategies. Because these models involve assumptions regarding operational and environmental conditions, the results are predicted impacts. Frequently, this is where the cycle of energy modeling stops with little attempt to verify model results: an open loop.
A False Comparison:
Comparing utility bills with results of the design phase energy models in an attempt to verify savings impacts can be a false comparison. Operational characteristics and environmental factors nearly always differ from initial assumptions.
Additionally, there is always the possibility the energy modeler or modeling program did not do a good job of representing your building. With an open energy modeling loop there is no feedback to verify whether the operation assumptions or the energy model itself was a good predictor, and thus, you don't truly know if the investment in energy modeling and efficiency had more or less value than anticipated.
Closing the Loop:
To close the energy modeling loop, both the proposed and baseline energy models need to be informed with actual measurements of building operation and the environment. Even in the case of renovations to existing buildings, where utility bills are available for both before and after improvements, there are still often operational and environmental factors that will affect this comparison. Only with Closed Loop Energy Modeling can you quantify the true value of your energy efficiency investments. The process not only provides a credible statement on a building's true performance but also informs building owners, operators and designers to make better future design decisions.