3.1. System Design—Scenario Development
In the following Figure 3
, the case study site is again displayed including the intended positioning of the heating centers and the anergy grid (both light blue), the pipe from the adjacent cooled office building (dark blue), the wastewater heat exchanger in the southeast (red), and the pipes from/to the geothermal boreholes (brown). Colours of buildings indicate their planned height (the darker the higher).
Taking into account the variety of available heat sources discussed before, different combinations can be considered. Furthermore, the evaluation of the contribution of fossil fuels was explicitly asked for by the City of Vienna for environmental and economic comparison. Four different combinations of the usage of the available renewable sources were compiled to form four scenarios plus a fossil, natural gas-based reference scenario.
All of the four scenarios include (1) solar energy in the form of PVT collectors respectively PV collectors, (2) wastewater heat, (2) ambient air heat in a quantity that balances the annual heat consumption together with the fixed amounts from the other sources as well, (4) waste heat from surrounding offices, and (5) geothermal boreholes for storing thermal energy. The scenarios differ concerning the dimensioning of heat pumps in the five heating stations between the low-temperature grid and the five local heat grids. Either they were dimensioned to the lowest expectable temperature in Vienna which is defined to be minus 12 °C expressed in a 2-day mean value, or the threshold was 0 °C (below this, gas heating boilers would cover a part of the heat load). As reference scenario, in order to compare the environmental and economic effects, a standard gas-condensing boiler with minimum solar thermal collector contribution according to the Viennese building law (1 m2
collector per 100 m2
gross floor area) was considered. The main assumptions of the five scenarios are summarized in Table 5
The heating and cooling energy is distributed via a two-pipe circular low-temperature (“anergy”) grid consisting of plastic material transporting water (without glycol). For heating purposes, pipe 1 serves as inlet and pipe 2 as return flow, whereas for cooling purposes, pipe 2 serves as inlet flow and pipe 1 as return flow. As the temperature difference between grid and soil is approximately only 10 °C, no extra pipe insulation was considered. As the temperature levels of heating and warm water are different, extra pipes have to be foreseen. The advantage of multiple heating centers is that the length of warm pipes can be reduced remarkably and that more extraction points of heat from the grid lead to a more stable grid. If extra pipes between the heating centers are foreseen, the resilience of the grid is increased as a breakdown of one heating center can then be (at least partly) compensated; however, in each heating center, a cascade of heat pumps has to be installed, therefore, the resilience of one single heating center is already relatively high. The estimated power and energy consumption per heating center is shown in Table 6
The area used for solar collectors is equal in all four renewable scenarios (30,000 m2
, about 7% of the total surface area); the difference is the technology used on this area (PV vs. PVT). The resulting annual energy output per m2
is 152.8 kWh/m2
a for electricity for PVT respectively 146.5 for PV and 450 kWh/m2
a for heat for PVT. From the master plan of the development area, a total gross area of 93,000 m2
was calculated. According to IEA [48
], the usable roof area is 1/3, i.e., 31,000 m2
. To be on the safe side, a value of 30,000 m2
was agreed (also considering area for heat pumps). A horizontal assembly of the collectors was chosen as lower wind force would occur, assembly is easier, and self-clouding would be minimized. For instance, the example of Suurstoffi (CH) shows that this design is feasible and favourable [20
For a first (rough) assessment of the wastewater heat potential, an analysis of the catchment area was carried out. Statistical data [49
] delivered an estimation of 23,900 inhabitants in the catchment area of the main sewer passing the investigated area. Moreover, a number of additional 4400 inhabitants of future “Nordwestbahnhof” residents were considered while offices, schools, etc. were excluded from the calculation. Applying a per-capita daily water consumption of 130 L resulted in an average dry weather wastewater flow of 42.6 L/s. For the first assessment, the local wastewater temperature was set equal to the one measured at the central WWTP (Table 7
). For estimating the heat extraction (energy) potential, a remaining minimum wastewater temperature of 6.0 °C was defined among the project and stakeholder group.
In addition to the rough estimation, to gain a better insight on and understanding of the available wastewater heat potential, wastewater flow and temperature were measured during a 5-month measurement campaign from 23 November 2015 to 28 April 2016. Figure 4
and Figure 5
display the monthly and total average dry weather wastewater flow and temperature from this measurement period.
Due to measurement problems, which could only be solved during December, Figure 4
does not contain the month of November.
The average diurnal flow patterns show a minimum discharge of about 20 L/s, a maximum of about 55 L/s and an average of around 43 L/s. This value matches quite well with the 42.6 L/s from the rough estimation (which was calculated with additional inhabitants on the Nordwestbahnhof quarter).
The average diurnal temperature patterns show a minimum slightly above 14 °C, a maximum of slightly above 17 °C, and an average temperature of around 16 °C. This value is even higher than the one measured at the WWTP and used for the rough analysis and thus highlights the importance of on-site measurements for more appropriate estimations of the available heat recovery potential. Anyway, for detailed planning purposes, these data are being considered a basic prerequisite.
The air heat pumps show a large potential which is hard to quantify in total, however, they were foreseen in an amount that secures a heat balance over the year (extraction from the storage equals feed-in over the year) and, therefore, their number depends on the specific scenario. If air heat pumps with 94 kW power are considered (large standard product), the number of necessary air heat pumps in the whole “Nordwestbahnhof” area is according to Table 8
The amount of waste heat from the office building was estimated to be 1.1 GWh/a based on data received from building users; for buildings to be erected in the development area within the Advisory Board, a consumption of 2 GWh/a was estimated. For the existing building, an extra pipe with a length of 130–150 m was necessary.
In total, 5439 boreholes with a depth of 100 m were necessary to close the time gap between production and consumption profiles. Forced cooling was applied with a temperature range between 6 and 22 °C.
Total energy consumption and generation for the different scenarios
As already expected, at the beginning of the research, the heating demand was much larger than the cooling demand. Therefore, additional heat sources were needed.
The potentials of the considered energy sources and the energy balances for the investigated scenarios are shown in Table 9
on an annual basis. The main result is that all heating and cooling demands can be covered by on-site renewable energy sources, even with skipping several sources as ground water and rivers.
To visualize the results, a graphical representation of energy fluxes for the example of Scenario 1, which is the preferred scenario according to the stakeholder discussions, is displayed in Figure 6
. This graph shows the low energy losses of the overall system that can be explained by the low temperature of the storage and amounts to only 6.3% of the energy supply.
It might surprise the reader that in all scenarios there is a “perfect” heat balance (demand and supply exactly the same). This is due to the dimensioning/number of the air heat pumps and the gas boilers, respectively. The potential would be higher but was cut at the point where it equals the demand.
Although warm water contributes to the energy consumption to a large extent, and seasonal performance factors of the heat pumps were set to be rather conservative, a total seasonal performance of almost 3.5 could be reached for Scenario 1.
While the thermal energy can be produced on-site to 100% (when considering the electricity share used for the heat pumps), electric energy can only be covered partially on-site. Only about one third of the electricity needed for the heat pump operation can be generated on-site.
The monthly energy balance of the seasonal storage is shown in Figure 7
. The overall annual balance is levelled out, i.e., the heat extraction equals the feed-in of thermal energy, securing a long-term stable ground temperature.
3.4. Presentation of Results to Decision Makers and Stakeholders
During the discussion after the final presentation of the results to the stakeholders, mainly the following topics respectively concerns were raised:
Reliability of the supply system: Some stakeholders had a bad experience e.g. from PVT collectors; additionally, the impact of the large number of geothermal boreholes on the geologic situation and the reparability were questioned. However, PVT collectors are the best way to make use of the limited area. The low number of PVT collectors compared to PV or solar thermal collectors might impair the service quality and the ability to differentiate between high-quality suppliers and others. Still, there are a lot of suppliers on the market already and examples show that the systems work. The geologic situation in Vienna is well known and groundwater areas which might be a threat to the system only occur at a certain depth interval.
Economic feasibility: One of the biggest concerns regarded, apart from the fact that the full cost analysis showed that the proposed system was not fully competitive, was the high initial costs. The long-term benefits were acknowledged, but as low payback times are expected, initial costs play a more important role.
Wastewater use: Due to recent practical experience, the Vienna sewer operator only allows external heat exchangers today, i.e., the wastewater has to be extracted from the sewer, filtered and fed into a heat exchanger in a separate building (or in the basement of an adjacent house on the “Nordwestbahnhof” area). Because of the large size of the planned plant, external heat exchangers are the better option anyway, as a heat exchanger in the sewer would have to have a size of several hundred meters.
Size of the project area: Due to the novelty of the approach, stakeholders were reluctant to implement the system in an area of this size. It was discussed to test the options in a smaller development area first. However, after some months, Vienna energy supply started to investigate the opportunities of using wastewater heat in Vienna in more detail. This shows that even if the concept might not be realized 1:1, as suggested in this study, the results still influence decision makers and some new elements regarding heating and cooling energy supply systems will be adopted.
3.5. Summary of the Results
The principle idea of this research was to design a grid suitable for heating and cooling of an urban development area based on non-fossil fuels. By inclusion of a seasonal storage, the cooling demand (predominantly in summer) would serve as waste heat potential to cover heating demand (predominantly in winter) and heating demand would serve as “waste cool” potential to cover cooling demand. However, the results show that in a development area with mainly residential areas in central Europe, the heat demand exceeds the cooling demand by far, which makes it necessary to exploit locally-available renewables and waste heat sources from surrounding buildings to reach a balance. Reaching this balance is crucial, otherwise a long-term cooldown of the seasonal storage (consisting of geothermal sondes and the attached ground) would occur. Areas with a higher share of industrial or other non-residential areas may have a more equal heating and cooling demand, which would be favourable for such supply systems.
However, due to a variety of locally-available renewable and also waste heat sources from the surrounding areas, it is possible to balance the deficit and to secure that the heat storage, which is charged with heat by a necessary amount. Most of the heat sources are typically available in urban areas and allow the conclusion that low-temperature grid-bound energy systems are a feasible way to heat and cool most newly built urban quarters. Solar, wastewater and outside air are renewable sources with a high energy potential available in virtually all urban areas. As most are low-temperature sources, the use of heat pumps and low-temperature distribution and dissipation technologies is necessary.
As expected, the environmental impact measured in primary energy consumption and greenhouse gas emissions is a lot lower than using fossil fuels. Compared to heating with gas boilers, reductions of 70 to 80% are achievable. The results moreover show that at current market prices, economic feasibility can be reached, if subsidies or taxes in a reasonable amount are taken into account. It has to be mentioned that in quarters with higher cooling demand, the economic situation would be more favourable, so that areas with a high share of industry or offices implementing such systems already likely show economic feasibility without subsidies.