Bradley Bathgate, PE
President and Practicing Principal at PWI Engineering
I was recently asked about the reliability of CHP at a hospital and what the thoughts were about having redundant units, known as N+1, or a standby CHP System. Although resilient and reliable at first thought, the ramifications of N+1 for constant power/thermal generation can be costly in the beginning and throughout the life of the plant. With CHP you are now integrating yourself more into other parts of the energy and infrastructure industries than before with the traditional systems. Let’s break things down to better grasp understanding.
For most hospitals where CHP makes sense, sizing a CHP Plant is based off of the waste heat availability so that all of the waste heat can be utilized to maximize efficiency, reduce operating costs and reduce emissions. Electrically speaking, the capacity of the generator at these sites is less than the minimum electrical demand that is required at the site each year. Even with the utility required protective relays, the generator/s has little chance to export power to the grid. When you introduce a redundant CHP System, although intended as a backup, multiple complications have been added to the project that if not known and understood will impact cost and schedule:
With regards to the electric capacity, if you have N+1, you more than likely now have more generation capacity connected to the normal bus than the minimum demand of the facility. This is magnified even more with gas turbines, where outputs are higher in the colder months when the electrical demand at a hospital is lower. Even though you only want to run one system at a time, the electric utilities usually approach these potential safety hazards conservatively and will require the project to install infrastructure for the utilities to maintain the level of necessary protection such as SCADA systems, switching, and Direct Transfer Trips. This can potentially add millions of dollars to the project and months if not years to the schedule depending on the approvals needed.
With regards to the fuel, primarily natural gas, the utilities have to consider delivering natural gas to ALL of the customers during the worst of conditions, taking tariffs into account. Naturally, natural gas utility companies take the planning conservatively. Some utilities understand the concept of redundancy, but most plan for TOTAL CONNECTED LOADS. Again, the natural gas utility can add unexpected costs and schedule delays for infrastructure upgrades that will be required on their side of the meter, to potentially include a new meter. If you want to change the service from interruptible to firm, that is an additional consideration. But it doesn’t stop there. The design of the system is also affected, especially if you have to consider natural gas compression. More than likely the MINIMUM GUARANTEED PRESSURE will be lower with an N+1 scenario, resulting in adding natural gas compressors to the project or oversizing of the natural gas compressors, affecting first cost, and affecting the lifecycle due to lower efficiency of the compression skid.
With regards to the reliability of the systems, if you have the proper team implementing your CHP project you will have the proven and reliable equipment manufacturers involved as well. Paying a little more up front on the equipment saves multiple times more on the operational costs with increased run time, maintenance programs, remote monitoring, etc on the back end. These systems do need maintenance, but are intended to run at 100% output all of the time with the exception of scheduled maintenance. Can they start, stop and modulate? Sure, but with increased wear and tear, loss of efficiency, and poor emissions. There are 8760 hours in a year, and these systems can easily obtain 8,300 run-hours at 100% output, and we have seen that number exceeded. Once it is up and running, let it run. So what happens to the N+1 just sitting there? You aren’t going to mothball it. It is not much different than the result you would get if you left your car sitting in the driveway for six months without running and maintaining it. Hold your breath when you turn the key (or now push the button).
If you do have heat recovery boilers and hot water systems, they need to be maintained, if not kept warm ready to fire. They need to be cycled and blown down in order to be kept operational. This is a loss of overall system efficiency and an increase in operational costs.
The gas turbines and reciprocating engines are full of lubricants. These lubricants need to be kept warm and the mechanical systems need to be turned over from time to time. Alternators need to be rolled. Seals and gaskets that are under high pressures need to be maintained. This too is a loss of overall system efficiency and an increase in operational costs. Imagine what would happen to emergency generators if they were not exercised/tested regularly.
The economics of the projects are greatly reduced. The capital to implement it increases with no increase in the savings. There is actually a decrease in savings as maintenance costs increase and system efficiency decreases. You will even find that the people selling the proven equipment will not recommend N+1 on these types of systems, because they know the project will struggle to get the financial approval, and if they do, they will struggle to be kept operational.
If the N+1 CHP system that was intended to run when the primary is offline cannot, be expected to be penalized on your natural gas and electric bills, as deviations from contracted volumes can create un expected budget overruns.
With more and more on-site generation being implemented, and the need for the utilities to continue to provide electricity in the event of a CHP system outage, we have also been seeing development of utility requirements for these CHP systems to have standby capacity, which will be more unexpected costs.
There are other factors that need to be considered regarding air permitting, procurement of utilities and the ability to bring the assets to the market and maximize the ROI. There are ways to understand and mitigate all of these potential risks from the beginning with a good Feasibility Study and Definition of Scope rather than finding out in the middle of the project. Please contact PWI Engineering if you are interested in learning more.