Design Progress March 2014
At the start of the biogas digester design process, the design team laid out 7 technical objectives to accomplish. These are itemized below. Over the last 3 years, significant progress has been made on meeting the first 5 technical objectives. In summary, the technical team has demonstrated that the biogas digester will function at Gorak Shep, the heat loss of the digester is known and there are available subsystems that meet the Gorak Shep requirements. The design team is refining the preliminary design that was briefed in Nepal in 2013 based on the inputs from this site survey. Major design efforts in 2014 are focused on the implementation of the biogas system and long term sustainability of the system at Gorak Shep.
The technical objectives were to technically answer the following questions:
- 1) Will an anaerobic biogas digester function at the low temperatures at Gorak Shep and using human waste as feed stock?
- 2) What factors influence the performance of the biogas digester?
- 3) What are the Gorak Shep design requirements?
- 4) What is the heat loss of the digester under various external temperatures, desired internal temperatures of the digester and different R-value insulation of the system?
- 5) What are potential design solutions to negate the heat loss and meet the system requirements for a digester at Gorak Shep?
- 6) How to implement the approved design?
- 7) How to provide long-term sustainability?
Since there are many variables that determine the quantity of gas that can be produced, a computer model has been developed that, based on the quantity of human waste input, water input, and digester size, quantifies the amount of methane gas produced as a function of internal digester temperature and retention time. The results from the model validate the technical feasibility of the project, but also allows trade offs of the input parameters to optimize performance and identify the critical factors in the successful operation of the digester in low temperatures.
More details about digester performance and temperature:
A mass-energy model for the biogas system has been developed including fuel preparation, digester, “green house” and effluent. Results show that the system is not sustainable without supplemental heat (green house and/or solar panels or other).
GORAK SHEP DESIGN REQUIREMENTS
- The system will maximize the use of locally available building materials and labor.
- The biogas system must be designed with off the shelf technology systems, easily maintained and sustainable.
- The system must be able to function during extended periods of no or limited solar energy.
- An average of 12,000 kg (13.2 tons) of human waste is generated at the climbers’ base camps at Everest, Nuptse, Lhotse and Pumori. This amount is divided between two climbing seasons: one in the spring and one in the fall. The spring season accounts for about 75% of the annual total waste.
- The slurry temperature needs to be maintained near 30°C for optimum methane production.
- The human waste from the base camps will need to be mixed with water to bring the waste concentration (percent of the effluent that’s solid) down to 8%.
- Pathogens remaining in the effluent from the digester must be safe to use as fertilizer.
POTENTIAL DESIGN SOLUTIONS
5 different design solutions were considered which used combinations of PV panels, solar heaters, immersion resistance heaters, electric water heaters, water storage devices, and pump driven heat transfer loops. The pros and cons of each system were presented to an outside technical panel of experts with a recommendation to proceed with a design (Preliminary Design 05/2013) that features:
- Heavy insulated (R-50) digester
- Heavy insulated (R-50) greenhouse with window for passive heat
- Insulated water tank heated by PV solar panels
- DC hot water pump to move heated water thru coils in the digester floor
- Simple drain field to dispose of effluent.
GORAK SHEP SITE SURVEY/CONCLUSIONS
A senior design engineer from the Mt. Everest Biogas team spent a month in Nepal and met with the tea house owners and porters at Gorak Shep; officials of Sagarmatha Pollution Control Committee (SPCC); Mt. Everest Foundation for Sustainable Development (MEFSD) and Biogas Support Program (BSP)-Nepal.
CONCLUSIONS FROM THE SITE SURVEY
- The teahouse owners and waste porters all support this project
- Water is available and drainage conditions are excellent for disposing of the effluent.
- The proposed site meets with the teahouse owners approval and:
- The sandy soil will help insulate the digester
- Risk of accidents by humans, animals or rocks is low
- Solar exposure is very good
The full presentation can be downloaded here:Presentation for BSP
TECHNICAL TASKS TO GO
- Refine preliminary design based on Gorak Shep site survey.
- Continue performance modeling of the digester and passive solar building.
- Acquire cost and schedule data on system components.
- Design a water collection system at Gorak Shep.
- Perform a risk assessment of the design/operation.
- Develop long term sustainability training and operations documents.