A guiding goal of an energy modeler is to construct a realistic projection of the energy use of a building. When one is working on the energy model of a Passivhaus commercial building, there are many issues that confront an energy modeler. These issues potentially include such things as: diverse occupancy profiles, large scale diversity in zonal heat gain and loss, accurate prediction of occupant schedule and process load accounting. This article is intended to open a discussion on the latter; process loads and their impact on PHPP energy calculations.
For the energy modeler, the first and arguably most basic understanding of any energy modeling tool is to understand what it is doing “behind the curtain”. Luckily, the PHPP is transparent in that respect. Second, and probably the most important item to understand, are the limitations of the tools being used. Here the PHPP has some very real shortcomings. To start with, as a static program, the PHPP has only rudimentary ability to look at a schedule of loads. I find that the built in “non-dom” or commercial side of the PHPP is adequate for generalized prediction of loads of building types with very static use, such as office buildings and school classrooms, but as soon as one crosses the threshold to a more diverse usage in buildings such as churches, theaters, school cafeterias or medical clinics, supplementary calculations outside of the PHPP are required for accurate load prediction. These calculations become critical in understanding both the interior heat gains (IHG) and primary energy (PE) of the building being modeled.
There are quite a range of topics that can and should be discussed regarding these calculations, but this article is limited in scope to process loads, so those other discussions will be left for another time. The first issue is defining a process vs non-process load. ASHRAE provides only limited guidance on the definition of process loads, stating that it is “the energy consumed in support of manufacturing, industrial or commercial processes not related to the comfort and amenities of the building’s occupants.” Not a very precise definition. For further definition one can look into the LEED definition, “The load on a building resulting from the consumption or release of process energy. (ASHRAE. (2010). ANSI/ASHRAE Standard 90.1-2010: Energy Standard for Buildings Except Low-Rise Residential Buildings. Atlanta, GA: American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc.)”, once again not very informative.
For the purpose of this discussion, process energy is considered to include all energy used above base usage for a typical commercial building. For commercial construction, this includes but is not limited to; equipment beyond the scope of general office and general miscellaneous equipment, kitchen cooking, kitchen hood exhaust and refrigeration, laundry washing and drying, lighting beyond general space lighting (e.g., lighting integral to medical equipment), industrial equipment, medical equipment and other items related to the specialized usage of the building (e.g., water pumps for process water, etc.).
Non-process or base energy includes general lighting (such as for the interior, parking garage, surface parking, facade, or building grounds, except as noted above), HVAC (such as for ventilation, space heating, space cooling, fans, pumps, toilet exhaust, parking garage ventilation, etc.), service water heating for domestic or space heating purposes, general office and general miscellaneous equipment, computers, elevators and escalators, small break room type kitchen cooking and refrigeration and general plug loads for small auxiliary electric such as copiers, printers, task lighting, etc.
Now let’s look at how process loads are defined in the PHPP. The PHPP energy definition is as follows:
“Total specific primary energy demand* ? 120 kWh/(m²yr)
* The primary energy demand includes the energy demand for heating, cooling, hot water, ventilation, auxiliary electricity, lighting and all other uses of electricity. The limits set above for the specific useful cooling demand and the primary energy demand apply for schools and buildings with similar utilization patterns.
These values are to be used as a basis but may need to be adjusted according to building use. In individual cases for which very high internal heat loads occur, these values may also be exceeded upon consultation with Passivhaus Institute. In such cases, proof of efficient electrical energy use is necessary.”
As far as PHPP is concerned, process loads are just another part of the overall energy load and the allowable amount of primary energy may be adjusted to account for the building type being certified. This definition has led to a prototype certification process, wherein best practices are defined by building type and each specific project type is examined on the basis of these best practices. While the certification criteria is an
interesting philosophical discussion, as an energy modeler I want to know how to accurately predict the energy usage and heat gain implications of process load.
In assessing process load, the first step is to create an accurate use profile for the equipment. This means constructing a schedule for each piece of equipment which includes their time of use, the energy used during operation, an approximation of the interior heat gain characteristics of the equipment (i.e., a vacuum pump with dedicated intake and exhaust will have less impact on IHG than one that draws intake and exhausts to a mechanical room within the envelope or an LED lamp has much less percent of its energy as radiant heat than a halogen lamp).
Creating an accurate use profile can be simple or complex, the following are examples of initial occupancy schedules for a simple and complex building occupancy pattern.
Determining the last two pieces of information, energy used during operation and an approximation of the interior heat gain characteristics can become challenging. Determining the actual energy usage of a piece of equipment can be a trying exercise in the real world. We have found that this is best accomplished through short term monitoring of similar equipment. Using name plate ratings will typically lead to overestimation of both energy and heat gain. Simple current transducers and mobile loggers make quick work of actual energy performance. The IHG of the equipment is a bit trickier. Because IHG depends on multiple factors including efficiency, process, sensible vs latent heat, etc. determining how much heat is released and how much of the heat release via radiant, convective or conductive heat there is a certain amount of “artistic finesse” in assessing IHG and assigning it appropriately to the building energy balance. For example, IHG in a commercial kitchen can be a challenge in the zone in which the kitchen is located, but in a 70,000 sq ft school with a 2,000 sq ft kitchen using the IHG from the kitchen to offset heat loss in an area far from the kitchen is a mistake without some means of actually capturing and moving that heat to another zone location. ASHRAE has some good general guidance for typical equipment IHG in the ASHRAE Handbook—Fundamentals. When one gets outside the ‘typical’ equipment, field determination of IHG becomes critical and more difficult.
Using the schedule of equipment now becomes the basis for determining process load effect on both PE and IHG. For PE calculation, simple multiplication of hours use by wattage can be made and the sum of the equipment energy can be entered into the Aux sheet in one cell which one can label “process load”. This will carry over to the electricity sheet and will be calculated in your PE tabulation. Please remember, if you have equipment that has different fuel sources than a separate tabulation will have to be made for these so that you can manually assign the appropriate fuel factors to the PE calculation.
Using the equipment schedule for IHG calculations takes a bit more time and a full understanding of the building and the building’s mechanical system. First, if the building warrants it, create a zone concept for the building; if the building does not warrant a zonal calculation, then this first step is not necessary. Next, a basic mechanical system concept is required to determine if and how IHGs will be distributed throughout the building. As we all know by now, buildings are dynamic holistic systems and without an initial mechanical system concept, the effect of the IHG on the energy balance of the building will be almost impossible to determine accurately, with any degree of confidence. Once a mechanical concept is determined, you can apportion loads to zones and begin to determine the peak heating and cooling loads of these zones. This is schedule dependent, for example in a theater with 300 people and 150 par lights, the peak cooling load will occur during a sold our performance in the summer using ASHRAE summer outdoor design conditions as a guide. The peak heating load will occur when the theater is empty in the winter once again using ASHRAE winter outdoor design conditions as a guide. As PHPP is a static model, we cannot easily model the space hourly, as we can with dynamic models, but determination of peaks is what will guide our system selection and sizing and lead us to accurate predictions of energy use. One uses the maximum and minimum IHG to predict these loads.
The last and perhaps most difficult item to model is HVAC energy use in PHPP for dynamic building types. In the case of fairly static buildings, standard office buildings, school classrooms, the PHPP is quite satisfactory, but in the case of diverse and dynamic profiles, I find there is a dichotomy between a “good enough for certification” model and an “accurate enough for my prediction” model. This is because in complex usage situations one must blend many different occupancy and use profiles (i.e. PHPP’s) to come up with a final static PHPP for certification. In these dynamic building types, the certification model is a general blending of the occupancy pattern to determine annual total energy use and IHG. But for detailed prediction we use a dynamic model calculation that is synced with the PHPP. While this is more time consuming, it allows for both a cross check and a realistic prediction of building performance.
While the PHPP has some limitations, with a bit of extra effort and thought, it can be used to simulate many aspects of process load in building analysis.