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Digital Energy's hourly simulation model DES-SIM/8760 breaks down the complexity of District Energy System (DES) into simple and logical steps. There are a number of sophisticated computer models available in the industry for individual elements of a DES like buildings only or chillers only or combined cycle plants only. However, DES-SIM/8760 is the only model of its kind that is able to wrap its arm around the "big picture" of all elements in an energy system to help evaluate the enterprise level energy picture from a Chief Financial Officer's (CFO) perspective.

Think of what a CFO of an organization needs to know regardless of plant complexity. Primarily, the cost of fuel, the cost of electricity and the cost of operations and maintenance. DES-SIM/8760 is designed to help get these answers through hourly simulation across a boundary that wraps around the ENTIRE energy system for that enterprise.

There are numerous examples of campuses and organizations that have a complex array of energy systems. In each case, optimization questions prompt the need for a sophisticated evaluation technique to get what the CFO needs to know, such as what is offered by DES-SIM/8760. Consider the two examples below that we ran into recently at Digital Energy, Inc.

Major Campus A - District Energy System

Campus A has a 21,000 Ton central chiller plant and a 42,000 Ton Hour thermal energy system that is served by another 5,000 Ton satellite plant. Chiller plant has a variety of chillers including electric driven centrifugal, steam turbine driven centrifugal and absorption type chillers. The campus uses a steam distribution system that demands as much as 200,000 PPH of steam. The overall electrical demand on the campus exceeds 50 MW. A 42 MW cogeneration plant consisting of two, 14 MW capacity LM 1600 Gas Turbines and a 14 MW induction/extraction steam generator currently service campus utility systems. Campus chilled water Delta T varies from 11° F to 18° F. The challenge faced by the campus - What is the optimum capacity addition required to enhance economics of the overall District Energy System?

Major Campus B - District Energy System

Campus B has two chiller plants, one rated at 3,000 Tons and another rated at approximately 2,000 Tons. The 3,000 Ton plant includes a 22,000 Ton Hour thermal energy system and a 1000 Ton absorption chiller that works in series with a 1,000 Ton electric centrifugal chiller. The chilled water Delta T varies from 12° F to 18° F. The campus demands as much as 45,000 PPH of steam. The campus has a 14 MW cogeneration plant consisting of two, 5 MW Solar Taurus gas turbines and a 4 MW condensing steam turbine. The 3,000 Ton chiller plant is old and campus is considering upgrades to the same. The challenge faced by the campus - What is the best combination of chiller types to be considered for the chiller plant upgrades to optimize economics?

Our Approach to Hourly Simulation of District Energy Systems

STEP-1 (Loads)

We start with understanding nature of your hourly loads that you experienced over the most recent 12 months. Loads may be chilled water (Tons), hot water or steam (Million Btu/Hr.) and electricity (kWh). If you do not have good load data, contact us to discuss how we may help derive the load data.

STEP-2 (Time of Use Pricing)

We study the time of use utility price structure for electricity that applies to the DES. Typically, most utilities have prices that vary as a function of summer and winter seasons. Within each season, rates are organized by On Peak, Mid Peak and Off Peak periods. Both energy and demand based pricing is relevant for simulation of DES costs.

STEP-3 (Equipment / System Configuration)

We want to simulate the DES system exactly as it is configured. So we will use the specific equipment you have like chillers, turbines, storage tank, etc., their capacity, rated efficiency, part-load characteristics as well as process and instrumentation (P&I) diagrams to simulate the exact process. We also note the existing sequence of operation of major systems and equipments so that our simulations over any hour accurately reflect the way you currently operate the plant.

STEP-4 (Monthly and Yearly Aggregation)

Once we build the hourly process logic for one hour, we extend the same over 365 days and 8,760 hours to capture all realistic conditions, including plant availability. As we simulate each hour, we continue to aggregate performance parameters as well as costs and savings on a monthly basis and time of use basis. The process helps visualize performance at both the Plant Engineer Level as well as Executive Level to get the big picture on an annual level economics.

Once we customize the hourly simulation model to mirror your DES, we can easily conduct sensitivity tests on loads, energy prices, sequence strategies, turn down strategies, process parameter changes, etc. Our approach eliminates guess work typically associated with the less rigorous “bin” analysis methods.
District Energy System for a Large Campus
Digital Energy, Inc. was requested to assist a large, world renowned educational institution to evaluate if it made sense to expand the plant capacity of an existing 42 MW cogeneration plant by adding a larger gas turbine at 34.7 MW in place of an existing 14 MW gas turbine. DES-SIM/8760 was used to answer the key question, reliably and precisely.

The DES system consisted of the following:

1. A central chiller plant consisting of two 5,300 Ton Steam turbine driven centrifugal chillers, that were discharging steam to four 1,500 Ton one stage absorption chillers. In addition to the steam turbine/absorber piggy back arrangement, the central plant has one 5,300 Ton electric centrifugal chiller.

2. The campus had a satellite chiller plant that was designed to produce 5,000 Ton of cooling that was both, capable of charging a remote 42,000 Ton Hour chilled water storage system and delivering capacity to a common chilled water distribution system.

3. The cogeneration plant consisted of two 14 MW GE LM 1,600 gas turbines, each coupled to a 100,000 PPH heat recovery steam generator with duct burner capacity of approximately 60,000 Lbs/Hr. each.

4. Excess steam from the steam generators was injected to a 14 MW steam turbine generator. Steam was extracted from the steam turbine to deliver 125 PSIG steam to the campus. When the chiller plant absorbers could not make use of all the steam from the steam turbine driven chillers, excess low pressure steam was injected into the induction port of the steam turbine to augment the power production.

5. Cooling towers were used to reject head from various chillers, steam turbine condenser, as well as various auxiliary systems supporting operation of the gas turbine packages and lube oil cooling systems.

6. Campus demand exceeded nearly 50 MW of electricity on warm days and cooling plant capacity peaked at nearly 26,000 Tons. On cold days, steam demand peaked at 180,000 PPH of 125 PSIG steam.

7. The key question faced by the campus - What is the optimal capacity expansion point to economically meet the current loads? Does a 20 MW increase make sense of is the sweet spot for maximizing economics something smaller?

Hourly simulations showed how a 10 MW plant capacity adder was all that was needed as opposed to a 20 MW plant capacity increase that was initially conceived. Having reliable and accurate findings in this case assisted the client in avoiding an expensive process of proceeding forward towards an economically risky direction.

Would you like to have a District Energy System Modeled? Are you looking to optimize your system's performance? Are you looking to make economical upgrades? Please contact our office and our DES expert will get in touch with you. CLICK HERE