The Electric Power Research Institute (EPRI), under the sponsorship of the Alaska Energy Authority (AEA), Anchorage Municipal and Light, Chugach Electric and the Village of Iguigig, conducted a study to investigate the feasibility of a technology known as River In-Stream Energy Conversion (RISEC) for Alaska river applications. RISEC technology converts the kinetic energy of water in free-flowing rivers into electricity by placing water turbines (similar to wind turbines) directly into the flowing water.
A total of six (6) river sites were selected for site assessment; the results are contained in Reference 1. After careful review, three sites were selected for conceptual level feasibility studies, the results of which are described in this report. The three sites were:
- Tanana River at Whitestone
- Yukon River at Eagle
- Kvichak River at Igiugig
This report describes the results of a system-level design, performance, cost and economic study of RISEC power plant installed at the three Alaska river sites of interest. Eagle and Igiugig are villages with isolated grid infrastructures, while Whitestone, near Big Delta, is located near a 26kV transmission line that would allow for a potentially larger-scale build-out.
Currently, RISEC devices are at a very early stage of development. In order to carry out performance, cost and economic assessments, EPRI established a baseline device design consisting of open rotor horizontal axis turbines mounted on a pontoon structure. Based on that baseline design, a parametric performance, cost and economic model was established to adapt the technology to the site conditions encountered at various sites of interest.
Cost estimates were cross-checked with data supplied by Verdant Power from their 5m diameter rotor design. While this proved a useful point of comparison, it is important to understand that Verdant Power’s machine is significantly larger in scale then the conceptual designs outlined in this report. As such, data could not directly be applied to this application, but was useful as a validation point for some of the model’s underlying assumptions.
The economic model used the simple payback period (SPP) as an indicator of the economic value of the potential project. SPP refers to the period of time required for the return on an investment to "repay" the sum of the original investment. For example, a $1000 investment which returned $500 per year would have a two-year payback period. It intuitively measures how long something takes to "pay for itself"; shorter payback periods are obviously preferable to longer payback periods (all else being equal). Payback period is widely used due to its ease of use.
The SPP for a RISEC power plant is the number of years it takes for the accumulated value of the revenues from the sale of electricity to equal the capital cost and the yearly operating and maintenance cost of the plant.
Iguigig and Eagle were treated as remote villages, and the RISEC plants were sized to meet a significant portion of the daily load (40kW for Iguigig and 70kW for Eagle). Whitestone was treated as a grid connected with a 26kV line that could likely be used to export more then 5MW. However, to be conservative, this study used a plant rated at 500kW. Any excess electricity produced is assumed to be absorbed by electrical resistive loads such as heating.
The value of electricity revenues is the avoided cost. For a rural Alaskan utility running on diesel, the avoided cost is essentially the fuel cost. With fuel costs of $8/gallon delivered and efficiencies of 13kWh/gallon, the avoided cost is typically 65 cents/kWh. The O&M cost of a diesel genset is 2-5 cents/kwh, but it was conservatively assumed that there would be no O&M savings. The following assumptions about escalation of costs were made:
Escalation of non fuel cost = 3% per year
Escalation of fuel costs = 8% per year 7
The results of this study showed that:
- As EPRI has found in previous ocean wave and tidal feasibility studies, economic viability of the deployment site is directly linked to the power density at the site.
- Rotor size for a horizontal axis turbine is limited by the water depth at the deployment sites. This limits the technology’s ability to scale a single horizontal axis rotor to higher power outputs.
- Power density peaks in Alaskan rivers occur during summer periods. This mismatch between resource availability and demand limits grid penetration. However some of this could be shifted by using electricity for alternative purposes such as heating.
- The commercial scale economics is limited in the isolated villages. Small deployment scales will yield higher comparable cost. This is not only true for RISEC technology, but is true for many other generation technologies as well.
- Small changes in the local velocities will create significant changes in power density since power density is a function of the velocity cubed. Detailed assessment of the local flow variations becomes a very important aspect of siting a RISEC device.
- Operational issues with this technology remains to be addressed with in-river tests. In particular, interference with ice, debris and wildlife need to be studied and, where required, mitigation measures incorporated into the RISEC device design.
- The SPP for remote village isolated-grid Iguigig is 3 to 4 years, for remote village isolated-grid Eagle 4 to 5 years, and for the remote village but grid-connected Whitestone case 8 to 9 years.
RISEC is an evolving technology field with different manufacturers pursuing different device concepts. Appendix B contains a list of developers active worldwide. It is included to provide the reader with an understanding of the range of technologies under development.