Solar for Multi-Dwelling Units in San Diego
Environmental Engineering Senior Design Project Group E4- UCSD, 2016
The benefits of going solar are well known by the public. Apart from greatly reducing carbon footprint and creating a sustainable environment, it drastically reduces the dependence on the electricity grid. This does not only drastically reduce the electric bills, but also protects against the unpredictability of rising energy costs and increases your property’s value. To date, most solar system owners reside in single-family units, where there is a cost effective proportionality between roof space and system output demand. Nevertheless, 60% of households are multi-dwelling units that are mostly yet to be introduced to solar technology. The United States holds approximately 5.6 million commercial buildings and 26.2 million residential renters in multifamily housing. Many of these buildings have large and unshaded rooftop space that are viable for solar photovoltaics or other solar products. To put this into perspective, the technical potential for rooftop photovoltaics in San Diego region will be about 4,691 MW (10,224 GWh) by 2020, in which 63% belongs to the residential sector. For comparison, in 2005, San Diego Gas & Electric had a peak demand of 4,058 MW and total sales of 19,214 GWh. Therefore, the technical potential surpasses over 100% of peak demand and represents 53% of total energy need(1).
(1)Anders, S., & Bialek, T. (n.d). Technical Potential for Rooftop Photovoltaics in the San Diego Region. Retrieved April, 2016.
(1)Anders, S., & Bialek, T. (n.d). Technical Potential for Rooftop Photovoltaics in the San Diego Region. Retrieved April, 2016.
Case Study-Central Park Towers
Abstract
This case study focused on the Park Central Towers located in Downtown San Diego. The building contains 38 units and is home to approximately 75 residents. The building was originally constructed in 1971, which is a major factor to consider when performing the roof analysis previously mentioned. The rooftop space is about 500 m2. However, after taking into consideration obstructions such as fans, the total usable roof area is 275m2, this is not enough area to provide energy to the entire building therefore the analysis was focused on covering the Homeowner Association’s energy use.
There were two analysis performed in order to assess the feasibility of installing a solar array: the structural analysis and the cost analysis. The structural analysis performed used the method of joint analysis, in order to determine the structure’s capacity and to withstand loads, and tilt angle analysis, in order to ensure the system’s capacity to withstand drag forces and changes in efficiency with respect to variance from the optimum tilt angle. The cost analysis used data collected from the National Solar Radiation Database and run through a MATLAB script giving the system’s potential energy output. Next, the system output was subtracted from the energy the HOA was using to determine the new electric bill and the revenue from selling the surplus energy. The saving on the electricity bill plus the revenue was taken to be the profits from the system and put into a Net Present Value Model to determine how long it will take to break even on the initial system installation. This method was re-run two additional times, once under the Virtual Net Metering and again under the new TOU Plus rates, to determine which model would be most profitable.
As a result, given the layout of the building, a total of 160 panels is projected to install. The solar panel model selected was the Sunpower E20/435 panel with 20% efficiency and four 18,000 Wh 3-phase, grid-tie inverter with the brand Schneider Optimum. The projected rated power of PV array was 69,900 Wh system and the cost of the system was projected to be $285,360 once installed.
The structural analysis deduced that the deflection on the bottom of the truss was approximately 0.8 inches, which is within standard allowable deflections for a flat rooftop within the city. Nevertheless, because the roof has already outlived its theoretical life, the roof will have to be rebuilt prior to installation so additional supports can be created. Furthermore, the cost analysis concluded that the shortest payback time was 18 years under the TOU plus rates.
There were two analysis performed in order to assess the feasibility of installing a solar array: the structural analysis and the cost analysis. The structural analysis performed used the method of joint analysis, in order to determine the structure’s capacity and to withstand loads, and tilt angle analysis, in order to ensure the system’s capacity to withstand drag forces and changes in efficiency with respect to variance from the optimum tilt angle. The cost analysis used data collected from the National Solar Radiation Database and run through a MATLAB script giving the system’s potential energy output. Next, the system output was subtracted from the energy the HOA was using to determine the new electric bill and the revenue from selling the surplus energy. The saving on the electricity bill plus the revenue was taken to be the profits from the system and put into a Net Present Value Model to determine how long it will take to break even on the initial system installation. This method was re-run two additional times, once under the Virtual Net Metering and again under the new TOU Plus rates, to determine which model would be most profitable.
As a result, given the layout of the building, a total of 160 panels is projected to install. The solar panel model selected was the Sunpower E20/435 panel with 20% efficiency and four 18,000 Wh 3-phase, grid-tie inverter with the brand Schneider Optimum. The projected rated power of PV array was 69,900 Wh system and the cost of the system was projected to be $285,360 once installed.
The structural analysis deduced that the deflection on the bottom of the truss was approximately 0.8 inches, which is within standard allowable deflections for a flat rooftop within the city. Nevertheless, because the roof has already outlived its theoretical life, the roof will have to be rebuilt prior to installation so additional supports can be created. Furthermore, the cost analysis concluded that the shortest payback time was 18 years under the TOU plus rates.
Current common area energy usage and cost
Currently, the common areas energy consumption is 73,600 kWh/year costing them $12,000. The monthly break down of the building’s energy usage and billing can be seen in Figure 4. The Park Central Towers apartments on the ALTOU rate which charges various electricity rates based on the time of the day and the season. As shown in Figure 2 and 3, energy used during “on-peak” periods (during the summer 11am to 6pm and during the winter 5pm to 8pm) cost more than energy used during “semi-peak” or “off-peak” periods. Summer On-peak, Semi-Peak, and Off-Peak rates are $0.10641, $0.09762, $0.07114 respectively. The Winter On-peak, Semi-Peak, and Off-Peak rates are $0.09759, $0.08326, $0.06354 respectively. Additionally, the final electricity bill will also include distribution and non-coincident charge. As a result, the cost savings will not be directly proportional to the energy savings.
Projected Layout
Park Central Towers was constructed while solar cells were first being brought into invention. The roof lay out is not catered to the installation of solar panels as there are air conditioning units spread throughout, as well as a large elevator shaft. Another factor that plays in is the space between each panel as we have them at an optimal tilt angle of 20 degrees. This spacing will allow each panel to be fully exposed to the sun and ensure there is no shading on the system. Spacing was also given for pathways to access each row of panels, as well as distance from the edge for safety, as well as shading concerns. All of these factors drop the 500 square meter potential of the roof down to a usable 275 square meters. In order to optimize the output of the roof given the surface area constraint, the most efficient solar panel was chosen.
The Sunpower E20/435 panel, as seen above, is 20% efficient which is a large gain for our system when the conventional panel is on average 14%. This allows for a peak power production of 435 Watts. These panels are dimensioned at 81.36” x 41.18” and are 56.0 lbs each. In addition to the proper selection of panel, it is pertinent to face your panels effectively. All of our panels will be South facing as that is the most efficient for this region. An exception to this is in the North East corner where we will face them to the West to capture more sun due to the elevator shaft shading. The inverter chosen was 18,000 Wh 3-phase, grid-tie inverter with the brand Schneider Optimum, as seen above In order to withstand the system load, there would be 4 inverters installed. Given the layout described above, the roof can support 160 panels, as seen in the project layout, giving us a 69,900 Wh system. The cost of a system this size utilizing the Sunpower panels is $285,360 once installed.
Structural Analysis |
Cost Analysis |
Recommendation & Conclusion
The above case study on the Central Park Towers served as an example of the bright future solar has throughout San Diego, and other solar rich communities. While it is not affordable today for such a solar system to be put in place, the future for such systems is very bright. Many solar incentives are available to the public, however, at this time it is catered to the need of single family homes. Not only do the incentives scale much more fortuitously to single family homes, but so do the paybacks of the solar systems. The drawback of Multi-Dwelling solar is inevitably due to the relationship of the available solar surface area to the amount of energy users within the unit. Change is on the horizon though. More efficient solar panels such as the Sunpower model are increasing the capacities of limited roof space, as well as making the payback duration more appealing for solar companies to invest in. Not only does the increase in technology help, but so does San Diego’s need for clean energy. The San Diego Climate Action Plan has a goal of making the city 53% renewable energy by 2020. In order to achieve this, it will take an effort not only for solar on single family homes, but taking advantage of all the roof space on multi-dwelling units across San Diego. Because of this policy that has been put in place by the City of San Diego, we will see an increase in state government funding, as well as incentive increase for solar county wide. Solar has taken single family homes by storm all across San Diego County. Multi-Dwelling Homes is the next opportunity for businessmen, and clean energy enthusiasts alike to come together to not only open up jobs and profit for local companies, but also contribute to the sustainable standards being pushed by the governing body of San Diego.
The above information is a product of the group E4- Megan Ong, Scott Fuller, Stewart Kerr, Luis de la Torre and Diana Wu Wong