Energy Management: A Story of Discovery
I was shocked to learn from the sales representative of a major HVAC equipment company that very few consulting engineers have used building simulation software to evaluate their designs. Fortunately,...
I was shocked to learn from the sales representative of a major HVAC equipment company that very few consulting engineers have used building simulation software to evaluate their designs. Fortunately, according to a recent evaluation from Natural Resources Canada’s Building Incentive Program (CBIP), the situation is changing because building developers are realizing that building simulation answers their needs. Here is a little story that proves how important it is for consulting engineers to learn and improve their skills in building simulation.
A few months ago I was busy working on various projects on what I thought was going to be a typical Monday at the office. Suddenly it happened: the phone rang. Granted this happens frequently for most consulting engineers, but this call was different. It wasn’t a call from client-ZYX requesting clarifications on the operation of his heating loop system, nor was it a call from the sales representative of company-XYZ asking to meet me. This phone call is what I like to call: The Big Call.
The Big Call is a good call: the kind of call that changes the course of things. It turned out that the person I was speaking to was from Alexis Nihon Group, a major building developer in Montreal. He wanted to know if I was available to help him make preliminary evaluations for a new office building in Gatineau, Quebec. “Yes! Of course, I am available!” I replied, already thinking to myself that I would have to reschedule urgent assignments and bring more work home. But for a 35,000-m2 10-storey building estimated at $30,000,000 I’ll make myself available! And so it began.
A first meeting was scheduled on Thursday of the same week in the office of Reliance Construction, which had been selected as design-builder for this project. This meant that I had only three days to prepare and go through a one-inch thick document on the requirements from the future building occupants: Public Works and Government Services Canada (PWGSC). I read through the document and quickly noticed that PWGSC were dictating the application of several standards and regulations to the new office building. The document gave details on what they expected regarding the quality of materials used, the type of elevators, the utilization schedule, the occupant density, the level of indoor air quality, the lighting level, the humidity level, etc. But what got me most excited was the special request, a challenge. Finally a good challenge! PWGSC wanted the energy consumption of the new building to be at least 25% below the energy consumption prescribed in the Model National Energy Code for Building (MNECB).
The first meeting was mostly an exchange to discuss our perception and interpretation of the project. The developer, the architect DCYSM, the construction company, myself representing Dessau-Soprin as mechanical-electrical engineers, and fellow consultans such as Schector Barbacki Shemie structural engineers were present.
We found out that the developer was invited to tender an irrevocable and competitive offer involving designing, building and then leasing to PWGSC usable office accommodation. The leased premises were also to be held for a minimum of 15 years. The winning bid for the project was the one that would result in the lessee paying the lowest annual rent. Unlike in most cases, the evaluation of the proposal would not only be based on the discounted first cost but also on the annual operating cost of the building, which includes energy, maintenance, cleaning, etc. Considering the operating energy cost of the building at the beginning of the construction project was certainly going to help improve the overall energy performance. We were heading off to a good start.
In preparing our bid, we considered various aspects of the design. We chose to use EE4 CBIP, a building software tool developed to evaluate the energy use of newly constructed buildings and verify if they meet the requirements of the CBIP program for the analysis.
Once the first building model is complete, anything is possible. Now that’s what I call experiencing the benefits of building simulation! We selected a range of different measures that would reduce the building’s energy consumption at least 25% below the MNECB. Then we used building simulation software tools to evaluate each component and decide which option was the most appropriate, based on their return on investment. Numerous simulations of the operation of the proposed building were completed and they gave very interesting results.
The project team decided on a building with large central office spaces. These spaces are not usually influenced by the outdoor weather conditions and very often require air conditioning all year long. To increase the energy efficiency of the building, their excess heat can be used to heat the peripheral space. A typical floor was 3,500 m2 and we considered 2,500 m2 of it as central zones with important heat gains. With lighting and equipment loads of 25 watts/m2 a simple calculation showed that the heat gain of a floor would be 87 kW. Considering the fact that the heat losses on the perimeter were 60 kW, there was more than enough heat from the internal zone to cover the peripheral zone. The remaining 27 kW would be used to heat ventilation air. The 27 kW generates enough heat if it is supplemented by a heat exchanger with an energy efficiency of 70% to recover heat from the exhausted air.
Heat recovery from internal heat gains eliminates the potential use of the so-called “free cooling.” It might come as a shock to some people, but free cooling is not as free as most engineers seem to think. This is because during free cooling mode, a large amount of dry outside air is admitted inside a building to decrease the internal heat gains. In fact, when a space requires humidity levels of 40% relative humidity (RH), the heat vapourization used to humidify the air may require more energy than the amount of energy saved during free cooling.
Our simulations showed that if the relative humidity level of the building’s spaces is kept at 40%, the annual energy use is 6,000,000 kWh per year. And if an electric chiller is used with the same indoor condition and without free cooling, the annual energy consumption decreases to 5,990,000 kWh per year.
The most important energy savings were to be achieved through the recycling of internal heat gain from the central zone to perimeter zones. Therefore for the design of the HVAC system, we decided to incorporate a double condenser (addition of a heat recovery condenser) and a low-temperature heating loop to recycle internal gains. This decision brought the annual energy consumption to 4,236,000 kWh per year. Figure 1 shows the energy use for cooling and heating for the three options. Because it is the heat gains that are being recycled rather than the indoor air, ventilation is kept to the acceptable standard
Even before we did energy simulations, one of the first aspects of the design the team looked at was the thermal resistance of the building envelope. Based on energy consumption analyses previously made on other office buildings, we decided that it would be very hard to design a more efficient building envelope that would improve on the standard set by MNECB. From an architectural standpoint, the requirements from MNECB and Quebec’s regulations on energy efficiency are quite stringent (see Figure 2). As a result, in the province of Quebec, increasing the thermal resistance of the envelope of a large building to improve its performance over MNECB is not cost effective. Any attempt to increase the thermal resistance of the building would delay the return on investment period. It was therefore decided that the thermal insulation of the building envelope (i.e. walls, roofs and windows) would simply respect the MNECB. A precast concrete wall with 90 mm polyurethane insulation was specified.
Although the performance of the building envelope will respect the requirements of the code, the building may still end up being less efficient if the envelope is no
t assembled properly due to unwanted air infiltration. Our solution was to use sprayed polyurethane to insulate the building. The main advantage of using sprayed polyurethane is that it fills any holes or defects and creates an airtight barrier that prevents any outside air infiltration.
We also analyzed the type of glazing. Two important factors have a major impact on the performance of a building: one on the energy consumption and the other on the size of the air-conditioning system. Windows with low-emissivity (low-E) glazing will decrease the energy use by increasing overall thermal resistance. Some low-E windows have a thermal resistance that is two times better than the thermal resistance of a standard double glazed unit. Windows that include inside shading devices or that have special reflective or tinted glass (better shading coefficient (SC)) will reduce the cooling requirement of a building. The analysis of the characteristics used to improve the glazing performance is in Figure 3. The final design incorporates glazing with a U-value (thermal resistance) of 1.70 and an SC factor of 0.25.
The choice of the energy source gave us very interesting options. Energy bills from Hydro-Qubec are calculated taking into account the highest demand (kW) and also the total of the energy use (kWh) between two consecutive readings of the meter (normally one month). The cost for the demand is $11.97 per kW. An additional cost of $12.78 per kW could be charged for each kW unit exceeding 133% of the subscribed demand (similar to the averaged demand for the year). The cost for the energy use is $0.0372 per kWh for the first 210,000 kWh for the month, and $0.0242 for each kWh exceeding the first bracket of 210,000 kWh. Other particulars are applicable but too complex for this discussion. The analysis of the tariff shows that the cost of electricity is related to the characteristics of the energy usage.
For the proposed building, we found that the use of electric baseboards for heating during the off peak hours will generate a saving of $9,400 per year when compared to natural gas hot water heating. But a saving of $20,000 per year could be realized by using natural gas for heating during the peak hours. An additional $8,800 per year could be saved if a high efficiency boiler were used instead of a standard 80% efficiency boiler.
Hydro-Qubec promotes the use of heat accumulators for peak heating. This equipment uses a mass of bricks that is heated with electricity during off-peak periods. The bricks can be raised to a very high temperature. The stored heat is extracted according to need during peak periods. A saving of $13,000 per year was calculated but the cost of the equipment was $130,000 and it created a load of 25 tons in the penthouse of the building. It was therefore decided not to use this option.
The building would use two 300 tons chillers. One of the chillers is equipped with a heat recovery condenser (double bundle). For the non-heat recovery chiller, the manufacturer proposed the use of an air-cooled condenser instead of the usual cooling tower. Though the initial capital cost of such equipment is less, the performance of the chiller is lowered by 50% and our analysis showed that the resulting annual additional cost would be $20,000, so this option was also rejected.
It was also proposed that we should use a differential of 8.33C on the chilled water and the condenser water instead of the usual 5.55C. Using the 8.33C differential, the analysis showed a small saving of $1,300 per year. For this particular proposal, a detailed analysis of the piping distribution should be done. The 8.33C differential could create an additional cost of more than $3,725 per year.
Energy efficient lighting fixtures will be provided throughout the building. Fixtures use T-8 fluorescent tubes with electronic ballast. A saving of 45% on the wattage density per square metre was found to be achievable on the electrical load and this is reflected with an annual saving of 14% on the total annual energy consumption.
When all was done and the design of the building was completed we achieved our goal and much more. The building is predicted to be 50% more efficient than the MNECB. For a large building of 35,000 m2 in Quebec the annual energy consumption would normally cost $550,000 a year. The 50% savings here means an annual saving of $275,000. By a far margin, the saving exceeds the cost of the analysis, and it could not have been done without the use of computerized energy simulations.
Our bid was successful and the project located on Boulevard de la Cit is now under construction and will be completed and commissioned by April this year.
Laurier Nichols, ing., is a consulting engineer with Dessau-Soprin of Montreal and led their team on building simulation. Marie-Judith Jean-Louis collaborated with him on the article.