Trigeneration for business centers, hotels and hospitals

For an ordinary business center, trigeneration (CCHP) pays back slowly. Cooling is needed for a short season, the constant load is small, and the investment in the engine, the chiller and the cooling tower takes years to return. In our model with 1 MW of cooling and 1,200 hours a year, the difference in capital costs pays back in 6.8 years. This is a case where we openly advise calculating a vapor-compression chiller first. Hotels, hospitals and buildings with a year-round load look better, and if the grid cannot supply the capacity at all, the calculation changes.

When does a business center not need trigeneration?

When the cooling season is short and electricity is needed mainly during the day. In the model, the electrical load without cooling is 400 kW and 3,000 hours a year, while cooling is 1,000 kW and only 1,200 full-load hours. With so few hours, the engine simply does not have time to earn back its price. Savings in the model are RUB 10.8 million per year, with capital costs for trigeneration of about RUB 104.3 million versus about RUB 30.0 million for chillers.

International methods know this limit. The US Department of Energy (DOE) holds that more than 6,000 operating hours a year is a good result, 5,000 to 6,000 calls for a detailed analysis, 3,000 to 5,000 may give an adequate payback, and fewer than 3,000 hours usually does not justify the investment. An example from practice: the BYU-Idaho university campus with a 4.5 MW gas turbine sent up to 45% of its exhaust to bypass on hot days (22% in a typical year), because in summer the heat had nowhere to go. The authors state plainly that a short cooling season lowers the payback of all options.

That is why we do not propose the scheme for an office building without a specific reason. The threshold for a lithium bromide absorption chiller: from 300 kW of cooling and from 3,000–4,000 hours a year. Below 2,000 hours it does not pay off.

When does it still make sense?

1. Electrical capacity is unavailable or very expensive. In the model with a grid connection (grid connection fee RUB 34.5 million), the payback shortens to 3.7 years. For a building that the grid has refused capacity, this is one of the few ways to get electricity.

2. There is a year-round load. A hotel, a hospital, or tenants working around the clock keep the engine running constantly. DOE names hospitals and hotels among the sites where absorption cooling works most effectively.

3. Tariffs are high. The sensitivity table shows a range from 5.1 to 12.5 years depending on gas price and tariff. The best payback of 5.1 years is reached at a tariff of RUB 12/kWh and gas at RUB 7/m³.

4. There is a winter heat consumer nearby: domestic hot water or a neighboring building.

Four questions before the calculation

If the answer to at least one is "no", calculate other schemes first.

Hospitals and hotels need backup

In a hospital, a cooling outage is unacceptable; in a hotel it is expensive. That is why the absorption chiller takes the base load, and an NCT chiller covers the peak and serves as backup.

Which building projects do we have?

The table shows examples, and many others are in the projects section.

Omega Tower cools three buildings: the business center itself, the Globo shopping mall and a fitness club.

Nobel Park is a 15-story Class A business center of 19,551 m²; the chiller runs on waste heat from a gas engine.

The Central Customs Administration building in Moscow lacked electrical capacity, so cooling was produced from microturbine heat. The chillers were delivered in sections and assembled in a basement with narrow passages. A letter from the energy center's general contractor thanks us for on-time delivery, rigging, installation supervision and commissioning.

The Smolensk academy gets hot water for the chillers from its own boiler house, although there is also an MTU gas engine on site.

As an example of Thermax chillers in operation, without our involvement: the Moscow Region Government House, three LT 42 T chillers running on heat from four Jenbacher JMS 620 engines, started up in 2006. Cold Energy (NCT) was not involved in this project.

What examples are there worldwide?

101 Miller Street cools 40,000 m² of offices and 10,000 m² of retail. Green Gold Complex runs on cooling in summer and on heating in winter, and a vapor-compression chiller stays there as backup. The 300-bed Sanjiban Hospital received a subsidy from India's Ministry of New and Renewable Energy.

What does the model feasibility study for a business center show?

This is a model calculation (tariff RUB 10/kWh, gas RUB 9/m³), not an offer.

The gas engine generates 1.3 million kWh a year at a capacity of about 500 kW. Waste heat covers 90% of the annual cooling demand. The shorter the season and the lower the constant load, the slower the investment returns. The full calculation and assumptions are on the feasibility study for a business center page. Typical reasons for low payback and the interfaces between engine and cooling are covered on the trigeneration mistakes page. If the numbers work, the engine and the cooling equipment can be ordered together from Cold Energy.

Submit a request for a feasibility study or check the numbers in the calculator: if the payback comes out longer than five years, we will tell you so.

Sources

• DOE, Absorption Chillers for CHP Systems, DOE/EE-1608, 2017: energy.gov
• DOE, CHP Resource Guide for Hospital Applications, 2007: energy.gov
• APPA, From Cogen to Trigen (BYU-Idaho), 2019: appa.org
• 101 Miller Street, Green Gold Complex and Sanjiban Hospital projects: Thermax materials

Frequently asked questions