Managing the Heating Demand in Institutional Buildings with an Innovative Thermal Storage System Alain Moreau, Eng, M.sc.A Senior Scientist Hydro-Québec Research Institute 600, avenue de la Montagne, Shawinigan Québec, CANADA moreau.alain@lte.ireq.ca Tom Steffes Product Development Manager Steffes Corporation 3050 Highway 22 North, Dickinson North Dakota, USA tsteffes@steffes.com ABSTRACT Hydro-Québec s Research Institute, in partnership with a heating system manufacturer 1, developed a new central electric heating unit that features thermal storage to allow its commercial and institutional clientele to manage their electricity demand more effectively. The new heating system incorporates two major innovations that have a significant technological impact in the area of thermal storage: 1/ it is the very first commercial heating system that can store heat up to a temperature of 900 C; 2/ it stores this heat in a brand new storage medium. Two versions of the product were developed: 1) a forced air version for customers with hot air heating systems; 2) a hydronic version for customers with hot water or glycol heating systems. The pilot projects that were conducted demonstrated that the new heating system considerably improves Hydro-Québec s clients electricity demand profile, allowing them to sometimes reach a weekly load factor of close to 100%. 1. CONTEXT In Québec, the power grid s peak period is strongly conditioned by the widespread use of electrical heating. To compensate for the increase in electrical demand, Hydro-Québec, the province s main electric utility, is constantly seeking ways to decrease the electricity demand of its existing clientele during peak periods. The use of electric thermal storage heaters appears to be a promising solution for Québec. The most common technology to date stores sensible heat in bricks up to a maximum temperature of 600 C. This technology is well honed, relatively affordable compared to other management solutions and has proven itself in several countries [1]. Traditionally, this technology has always been applied in the residential sector. In Québec however, the residential market is unattainable on the mid-term because electricity in this sector is not billed based on a time-ofuse rate and installing TOU meters in Québec would prove to be very costly. On the other hand, Hydro-Québec s commercial and institutional clientele has always been billed based on their monthly energy consumption and maximum demand. These customers therefore have an immediate interest, based on the current rate structure, in managing their demand more effectively in order to reduce their energy bills. In general and as a rule, the peak demand in these two sectors coincides with the grid s peak demand. By managing their demand more 1 Steffes Corporation, 3050 Highway 22 North, Dickinson, North Dakota, www.steffes.com
effectively, Hydro-Québec s commercial and institutional clientele will indirectly contribute to the management of the grid s peak demand periods. In the commercial and institutional sectors, the most commonly used electric thermal storage heaters use water to store heat (pressurized or non-pressurized water tank). However, because of the high level of heating required in these sectors, the thermal storage units designed for commercial and institutional customers are often very large and expensive, which constitutes a major barrier to their being installed in certain buildings. To offset this barrier, Hydro- Québec s Research Institute developed a new thermal storage heater that is able to store heat at a much higher storage density than other heaters available on the market. 2. DESCRIPTION OF THE NEW THERMAL STORAGE HEATER 2.1 Choosing the Storage Medium To ensure that the new thermal storage heater would be as compact as possible, the Research Institute worked towards developing a storage medium that would provide a high-level of storage density by volume. In addition to this criterion, the new medium had to 1) be reliable over time, 2) be safe, 3) be affordable, 4) provide an effective thermal conductivity to store heat all the way to the medium s core and 5) allow to easily recover the heat that is stored. At the onset, we considered storing sensible heat or latent heat of fusion (liquid/solid); for technical reasons, we eliminated thermochemical storage or liquid to gas phase change storage. Based on the above-mentioned criteria, including storage density, we finally opted for sensible heat storage. More specifically, the medium that was developed is a high-density brick that is able to store heat at 900 C. Among other things, the new brick contains hematite (Fe 2 O 3 ), an affordable material that is an excellent heat conductor. The raw material used to make the brick is based on the residue of a manufacturing process. This new brick therefore makes good use of a waste material. The brick s density is 4.2 g/cm 3, which is 10 to 25% higher than the bricks currently used in electric thermal storage heaters. Because of its high density, the brick can store more heat per m 3. In addition, being able to store heat at a temperature of 900 C was a huge technological breakthrough since thermal storage heaters were limited to 600 C. A new electric element was then developed to allow the unit to operate at such a high temperature. Taking into consideration that the specific heat of the new brick is 0.92 kj/kg/ C, its heat storage density is 859 kwh/m 3 2. Figure 1 compares this storage density to the density of several other categories of storage mediums, particularly phase change materials. Reference [2], which lists several categories of storage mediums, was used as a database to prepare the figure. In Figure 1, the phase change materials are divided into 2 groups, depending on whether the melting temperature is below or above 400 C; 53 different materials are included in the first group and 84 are included in the second. As shown in this figure, the new brick provides one of the best heat storage densities (kwh/m 3 ) among all the mediums that were listed. 2 When considering a variation in temperature of between 900 and 100 C.
For comparison purposes, this storage density is 12 times higher than the storage density of water (considering that the temperature of water varies between 40 and 100 C). Also, as compared to the storage density of existing electric thermal storage heaters, the new brick s storage density is close to 2 times higher. In order to obtain storage densities that are comparable to the density of the new brick, phase change materials where the melting temperature is relatively high must be used, for example NaCl (T fusion =801 C), MgCl 2 (T fusion =712 C), Na 2 CO 3 (T fusion =854 C). However, the production of thermal storage heaters that use these types of materials is not straightforward because, contrary to bricks, phase change materials must be contained. In addition, there are issues related to the segregation and potential safety hazards of many phase change materials. Although these problems can be addressed in cases where the storage is coupled with a heat plant (for example, a solar power plant with concentrators), the issues are difficult to resolve when the thermal storage heaters are intended for buildings. Lavigne [2] studied the possibility of further increasing the new brick s storage density by doping its pores with NaCl and KCl. This technique increased the theoretical storage capacity by 14.4% with KCl and by 16.7% with NaCl. The process revealed that it is feasible but that impregnating the pores of the brick with these phase change materials is not an easy task. Figure 1: Storage Density of Families of Storage Mediums 2.2 Operating Principles of the New Thermal Storage Heater The new thermal storage heater is a central heating unit designed to be installed in the mechanical room of buildings. Two versions of the product were developed: 1) a forced air version for customers with hot air heating systems; 2) a hydronic version for customers with hot water or glycol heating systems. Both versions use the same energy storage technology, except that the heat transfer fluid to provide heat to the building is different depending on the model. Hot air is used in the forced air version, whereas water or a mixture of water and glycol is used in the hydronic version. A schematic view of both models is shown in Figure 2.
Storage medium Electric element Extracting fan Hot water Cold water Air/water heat exchanger i) Forced Air Version ii) Hydronic Version Figure 2: Schematic Views of the Two Models During the client s or the utility s off-peak periods, the electric elements in the storage mass are activated to heat the bricks up to the desired temperature. Conversely, the elements are deactivated during the client s or the utility s peak periods. In order to recover the heat that is stored in the mass, a variable speed heat-extracting fan circulates air through the storage mass in channels between the bricks. The air enters the storage mass through the top and exits at a very high temperature through the bottom. In the case of the hydronic model, the hot air emerging from the storage mass then circulates through an air to water heat exchanger and heats the hydronic fluid circulating in the exchanger. This fluid is used to heat the building. In the case of the forced air model, the hot air emerging from the storage mass mixes with new air in proportions that will allow to reach the desired temperature mix to heat the building. In both cases, the speed of the variable speed fan varies so that the temperature of the heat transfer fluid heating the building is at the desired value. The forced air version is available in 5 models, whereas the hydronic version is available in 2. The power of the electric elements ranges from 53 kw to 160 kw, depending on the model. Table 1 shows the main features of each model. The most popular model is currently the 80 kw electric hydronic version. Because of their high storage density, the new thermal storage heaters are relatively compact. For example, the 80 kw electric hydronic version only takes up 1.1 m 2 in floor space and is approximately 2.5 m high. Table 1: Main Features of the Available Thermal Storage Heater Models Forced Air Hyd roni Model Electric Elements (kw) Storage Mass (kg) Dimensions (L X D X H) (cm) 53 kw electric 53 1,484 152 X 129 X 175 80 kw electric 80 2,225 152 X 129 X 229 106 kw electric 106 2,968 238 X 129 X 175 133 kw electric 133 3,709 238 X 129 X 229 160 kw electric 160 4,450 238 X 129 X 229 53 kw electric 53 1,484 86 X 129 X 195 80 kw electric 80 2,225 86 X 129 X 249
2.3 Energy Performance of the New Thermal Storage Heater The heating capacity of a thermal storage heater increases according to the thermal mass that is in the unit. In the case of the 80 kw electric hydronic model, which is the model that is currently the most popular, Figure 3 shows the evolution of the thermal output that can be delivered by the unit when all the electrical elements are deactivated and when, at the beginning, the storage mass is at its maximum temperature of 900 C. These curves were obtained by conducting an energy balance on the heat transfer fluid entering and exiting the storage heater. Figure 3: Thermal Performance Curves of an 80 kw electric Hydronic Thermal Storage Heater As shown in this figure, the 80 kw electric hydronic unit is able to deliver a constant thermal output of 40 kw thermal over a period of approximately 7.5 consecutive hours by simply drawing its heat from its storage mass. Over this period of time, the thermal output can be maintained at a constant value even if the temperature of the storage mass changes because the variable speed heat-extracting fan compensates for a decrease in the temperature of the mass by increasing the airflow in the mass. After a period of 7.5 hours however, the thermal output delivered by the unit gradually decreases because the storage mass reaches a temperature that is too low to maintain the thermal output that is desired. When this occurs, the variable speed fan operates at maximum speed. In order to prolong a constant output of heat, two options came to the fore when the new storage heater was initially designed: 1) increasing the heat exchange surface in the storage mass between the air and the bricks, which would have resulted in an increase in the overall dimensions of the unit; 2) choosing an air to water heat exchanger with a larger exchange surface, which would have resulted in an increase in both the cost and dimensions of the unit. Technical and economic constraints led us to opt for the current configurations. As should be expected, the period with the constant thermal output is longer when the unit delivers heat at a lower rate. As shown in Figure 3, the heater will deliver a constant thermal output of 28 kw thermal over a period of 12 consecutive hours, whereas the period diminishes to 3.5 consecutive hours when the heat is delivered at a rate of 65 kw thermal.
Tests similar to those illustrated in Figure 3 were conducted on all the models of the new thermal storage heater. Overall, they demonstrated, among other things, that the energy performance curves of the hydronic and forced air versions of units of the same size are very similar, except that the duration of the forced air models constant thermal output is approximately 30 minutes longer than that of the hydronic models. 3. EXAMPLE OF AN APPLICATION Prior to putting the new thermal storage heater on the market, demonstration projects were conducted in order to, on the one hand, test the unit in real-life situations and, on the other, demonstrate the new unit s energy management potential. Figure 4 shows one of the buildings that took part in the demonstration projects. The building houses community centre in Québec City. It is occupied between approximately 8am and 10:30pm. This 3,839 m 2 building was built in 1923 and, prior to the demonstration projects, was heated with a 225 kw electric boiler. Figure 4: Building where Tests were Conducted Two 80 kw electric hydronic thermal storage heaters were installed in the building s mechanical room replacing the existing electric boiler. The choice of these two models was based on an evaluation of the client s energy needs and on optimizing both the technical and economic aspects. The two thermal storage heaters are the building s sole heating systems, meaning that they should be able to heat the building and store energy at the same time. Figure 5a shows one of the storage heaters installed in the building. Figure 5b is a diagram of how the two units were connected to the building s hydronic system.
a) Hydronic Thermal Storage Heater b) Connections to the Building s Hydronic System Figure 5: Views of the Installation The capacity of the two thermal storage heaters to ensure the constancy of the building s electricity demand is shown in Figure 6a. It presents the building s overall demand profile over a period of 5 consecutive days (from January 29 to February 2, 2007). Figure 6b, on the other hand, shows the evolution of the temperature in the storage mass of the two units over the same period of time. In Figure 6a, the yellow section represents the building s demand for miscellaneous usages other than heating (lighting, domestic hot water, ventilation, etc.) whereas the red section represents the electricity demand of the two thermal storage heaters. Over the 5-day period, the average outdoor temperature was -16 C, with a minimum of - 25 C. Figure 6a shows that the overall demand of the building was constantly maintained to approximately 160 kw over the 5-day period, thanks to the two storage heaters. For comparison purposes, the building s maximum demand prior to the installation of the thermal storage heaters was 400 kw. Thus, depending on the demand for miscellaneous usages, the electric consumption of each of the thermal storage heaters was modulated so that the total demand never exceeded approximately 160 kw. Generally speaking, the energy input of the storage heaters was higher during the night because the electric demand of the other usages was lower during this period. During the night, the thermal storage heaters were also able to store heat in their storage mass for subsequent heating of the building. Globally, the usage factor UF 3 of the building during the same 5-day period was 91.5%, which is exceptionally high for an all-electric building. 3 The usage factor UF is the relationship between the average and the maximum demand over a period of time.
a) Demand Profile b) Storage Temperature Profile Figure 6: Test Results of a Demonstration Project 4. CONCLUSION The new thermal storage heater developed by Hydro-Québec s Research Institute, in partnership with a heating system manufacturer, is an important technological breakthrough in the area of thermal storage. Indeed, it is the very first commercial unit able to store heat up to a temperature of 900 C. Its storage density is 859 kwh/m 3. To our knowledge, this energy storage density is the best among all the storage heaters available on the market. The new thermal storage heater is currently being marketed in North America. One of the constraints imposed when the storage heater was being developed was that it must be commercially viable, without any subsidies, on the basis of its payback period. The many demonstration projects that were conducted with the unit have, in effect, shown that the return on investment occurs within less than 5 years by allowing the client to reduce his billable peak demand. The new thermal storage heater has also proven to be an interesting tool for public utilities allowing them to limit their clients demand during the grid s peak periods. It is therefore a win-win technology where both the client and the electricity provider reap the benefits. REFERENCES 1. Dumont, E., Moreau, A., 1996, Experimental And Numerical Study of Room Storage Heater Performance, Canadian Electrical Association. 2. Lavigne, K., Galanis, N., Moreau, A., 2006, Sensible And Latent Heat Storage Medium For High Temperature Electrical Thermal Storage, International Heat Transfer Conference Conference 13.