In the context of research, modeling has been conducted on various multi-apartment buildings in Latvia, considering different solar energy systems such as solar collectors, photovoltaics (PV), and their combinations. To broaden the scope of the analysis, the study focused on apartment buildings in Riga specifically, including those with five and nine story, because these types of buildings are the most common in Riga micro-districts. Energy profiles were developed for the reference 5- and 9-story apartment buildings: electricity consumption profiles and domestic hot water load profiles. Reference building parameters (based on real energy consumption data) shown in Table.
|No. of storey
|Roof area, m2
|No. of inhabitants
|Electricity demand, MWh/year
|Heat for domestic hot water system, MWh/year
Referenced 5 story multi-apartment building has 75 apartments. Calculated total electricity consumption of multi-apartment building for the year 2022 was 102,06 MWh, common use electricity consumption was received in real terms from a common use smart meter and was about 8.0% (8,17 MWh) of the total electricity consumption of building, calculated electricity consumption of 75 apartments – 93,89 MWh. The average electricity consumption of one apartment was calculated – 1252 kWh/year.
Referenced 9 story multi-apartment building has 36 apartments (88 inhabitants). Calculated total electricity consumption of multi-apartment building for the year is 72,87 MWh, common use electricity consumption about 17.36% (12.65 MWh) of the total electricity consumption of building, calculated electricity consumption of 36 apartments – 60.22 MWh The average electricity consumption of one apartment was calculated – 1672,83 kWh/year.
In order to determine the optimal size of the solar energy system and the self-consumption – surplus – deficit of generated/consumed energy, the hourly and monthly load profiles of the reference multi-apartment building were analyzed.
Based on previous studies related to the solar collectors’ integration in heating systems in the similar to Latvia climatic conditions, it was decided to focus only on the preparation of domestic hot water, since the proposed system is intended to be used on the multi-apartment building roofs in urban environment. Therefore, there will be no large areas for installing solar collector plant, as well as free land plots for installing seasonal heat storage tanks; the only technically and practically accessible option is to equip the system with small above-ground storage tanks.
The actual energy consumption data for the refenced buildings were obtained from the annual reports of AS “Rīgas Siltums”. Considering the regulations of the Cabinet of Ministers no. 222 (08.04.2021) “Methods for calculating the energy efficiency of buildings and regulations for energy certification of buildings” Article 15: “If, when performing energy certification for a building at the construction project stage or in operation, domestic hot water consumption data are not known, they shall be determined in accordance with the standard LVS EN 12831-3 :2020 “Energy efficiency of buildings. Heat load design calculation method. Part 3: Characterization of heat load and requirements of domestic hot water systems. M8-2 and M8-3 modules” Appendix B.2.” The operating schedule was determined in accordance with LVS EN 15316-1:2017/NA:2020, NA Table 3, in accordance with the Guideline for the application and implementation of the methodology for calculating the energy efficiency of buildings. The calculated thermal energy required for hot water consumption in multi-apartment residential buildings is from 19.8 to 48.2 (on average 34.5) kWh/m2 per year. Hourly domestic hot water consumption data calculated according to LVS EN 15316-1:2017/NA:2020 NA and EN 12831-3:2017 “Energy performance of buildings – Method for the calculation of the design heat load – Part 3: Domestic hot water systems heat load and characterisation of needs, Module M8-2, M8-3”. The domestic hot water consumption profile is compiled based on real heat consumption data for domestic hot water preparation and nPro software tool (for planning building and district energy systems) provided residential building domestic hot water load profile.
 JSC “RĪGAS SILTUMS” is the main supplier of heat energy in Riga. It performs the production, transmission and sale of heat energy, as well as provides technical maintenance of the internal heat supply systems of buildings that use heat energy.
Multi-apartment buildings vary in size, cardinal orientation, and the presence of objects that can cast shadows, so it is difficult to obtain a general rule for the optimal size of solar energy systems. Each building’s unique load profiles and production potential influence its consumption and therefore its economic performance. This report will discuss the different types of RECs to make it easier for the prosumer to select the type and size of system. For these purposes, the report provides a relationship between load profiles and the performance of the considered technical solutions for RES systems. To obtain more realistic data, the study is based on real building energy consumption data and analysis of simulated energy supply systems.
For the integration of solar energy into the energy systems of 5-story multi-apartment buildings, 3 system variants (technical solutions) were developed: PV, PV + solar collectors and solar collectors.
The 1st proposed solution is a rooftop PV system connected to the power grid
As a result, a PV system was introduced with 148 PV modules, the annual energy produced of which is 41,249 kWh. According to the calculations, in our case, the average annual self-consumption rate of a 5-story referenced building stands at approximately 40,42 %.
The 2nd proposed solution rooftop solar energy combined system: PV + solar collectors
The 3rd proposed solution rooftop solar collectors’ system
Solar collectors’ system was introduced with 144 flat-plate collectors, collectors’ area 288 m² with total solar fraction 48,5 %, the total annual field yield of system is 104,183.6 kWh. In order to ensure uninterrupted operation of the system, the system was equipped with stand-by storage tank (1500 l) and TES (23000 l), as well as the multi-apartment building was connected to the city’s centralized heating system, as heating is also required in the winter months. According to the calculations, in our case, the average annual self-consumption rate of consumed DHW of 5-story referenced building stands at approximately 51.51 %.
The 4th proposed solution rooftop PV system (for covering building electricity demand for shared needs)
Due to the small area, the roof of a 9-story building is less attractive for PV installation and more the specific design of the building’s roof with an elevated technical shaft for the elevator significantly influenced the placement of PV panels, reducing the usable roof area and decreasing the efficiency of the simulated systems due to shading caused by the technical structure. During the simulations it was decided to consider the possibility of covering the electricity demand for shared needs by PV generated electricity.
Polysun software was used for optimization of rooftop PV systems in multi-apartment buildings. A building model was created for simulation, and the PV systems were modeled according to the actual roof configuration.
Various configurations of PV systems were modeled for optimization, aiming to fully utilize the available rooftop area of the building. This included different orientations of PV panels (east-west, south), various installation angles ranging from 15 to 45 degrees and different installed capacities such as 8 kW, 16 kW, etc. The specific design of the building’s roof with an elevated technical shaft for the elevator significantly influenced the placement of PV panels, reducing the usable roof area and decreasing the efficiency of the simulated systems due to shading caused by the technical structure. Two optimization scenarios were selected for the PV system: Case 1 with a PV generator power of 16.56 kW and a PV generator area of 78.27 m2, and Case 2 with approximately 28.98 kW and 136.98 m2.
Based on the data provided by the modeling software for this real building located in Riga, Latvia, with corresponding coordinates (location not disclosed for data privacy reasons), in Case 1 the amount of electricity generated by the PV system is 13,889.12 kWh per year, and in Case 2 it is 24,251.48 kWh. Overall, in Case 1 the PV system can cover 19.06% of the total electricity demand (apartments + shared needs), and in Case 2 it can cover 33.28%. The annual electricity consumption of the building for shared needs is 12,648 kWh. Both systems can fully cover the consumption; however, a larger system will be more expensive. Therefore, a more optimal PV system was chosen.
For the optimization of the PV system, it is necessary to conduct a detailed assessment on a monthly and hourly basis, as solar energy exhibits pronounced seasonal and daily variations. An hourly profile for the electricity consumption needs of the multi-apartment building has been created and compared to the hourly production capacity of the PV system.
Nevertheless, there is usually a temporal misalignment between the intermittent generation of solar PV electricity and household electricity consumption. The PV system cannot provide a continuous and stable supply of electricity to the multi-apartment building throughout the year and day. To ensure a stable power supply to the facility using the PV system, solutions for base loads need will be: installation of energy storage systems (ESS) or connection of the PV system to the power grid. The costs of energy storage systems are still high and can double the payback period for the installation of panels . In our case, the optimal solution is to connect the PV system to the grid using the NETO settlement system. This allows the electricity generated on-site but not immediately consumed to be stored and used when needed covering property electricity consumption during times of low solar intensity. After performing calculations for a PV system with an installed capacity of approximately 17 kW (Case 1), taking into account electricity prices for December 2023, the average payback period will be 5-6 years.
Technical solutions for distributing the energy of solar PV panels in an apartment building
The distribution of solar PV panel energy within apartment buildings has earned significant research attention due to its potential to enhance sustainability and reduce electricity costs. Studies have investigated various technical solutions, with direct and indirect connections emerging as prominent strategies. Direct connection involves delivering solar power directly to consumers, optimizing energy utilization and potentially leading to cost savings. However, challenges such as securing consent from power distribution services, inverter regulation complexities, and control system uncertainties have been noted. In contrast, the indirect connection approach focuses on grid synchronization for power balancing and offers flexibility in battery integration. Challenges similar to direct connection are encountered.
Advantages and Disadvantages
|Electricity Cost Savings
|Consumers do not pay for solar power if ST consent is obtained
|Similar cost-saving potential if ST consent is obtained
|ST Consent Requirement
|ST may not grant consent for meter bypass
|ST may not grant consent for meter disconnection
|Optimized Energy Utilization
|Solar power directly supplied to consumers, minimizing grid reliance
|Grid synchronization for power balancing ensures optimal use
|Inverter Regulation Constraints
|Inverter’s power regulation may not match consumption demand
|Inverter’s power may not provide enough for all consumers
|Battery Integration Flexibility
|Choice to integrate batteries for energy storage
|Option to opt for a conventional inverter without batteries
|Complex Control in Battery Depletion
|Unclear control when the battery is depleted, and solar generation is insufficient
|Unclear control when inverter power is insufficient for all consumers
|Enhanced Grid Independence
|Reduces reliance on the external grid, promoting self-sufficiency
|Individual Cord and Meter Requirement
|Requires individual cords and two meters for each consumer
|Potential for Net Metering
|Opportunities for excess energy to be fed back into the grid, potentially earning credits
|Equipment must be carefully selected for compatibility
|Distributed Energy Generation
|Solar energy production is distributed across the building, reducing transmission losses
|Programmable Logic Controller (PLC)
|PLC installation and programming required
|Reduced Carbon Footprint
|Lower reliance on fossil-fuel-based grid power leads to decreased greenhouse gas emissions
|Data center or cloud service required for consumption data management
|Improved Resilience and Reliability
|Provides a decentralized source of power, enhancing reliability during grid outages
|Requires skilled programmers for development and maintenance
|Community Engagement and Sustainability
|Fosters a sense of community involvement in renewable energy initiatives
|Internet and Electrical Connections
|Requires at least two internet cables per apartment and specific electrical switchboard
|Product development may require debugging and iterative enhancements
|Requires independent electricity and internet connections, incurring additional costs