Case Study: Energy-efficient Technologies combined with modern DHC Concepts

Location

California State University, Fullerton

Case Study Significance

This study examines the environmental benefits associated with the upgrading of an existing DHC plant at California State University, Fullerton. These benefits, which include dramatic reductions in plant emissions particularly NOx, elimination of the use of refrigerants having non-zero ozone depletion potential, and substantial energy savings, are all indirectly a result of the centralized plant being readily and cost-effectively capable of implementing developing technologies and modern operating concepts. This study is a summary of a report entitled "Thermal Energy Storage, Energy Conservation and DHC at California State University, Fullerton" authored by Henrikson.

Project Background

The California State University, Fullerton campus was initially built during 1961 - 1963 and consists of twelve major buildings of approximately 150,000 m₂. The additional buildings are scheduled to be constructed between the present time and the year 2000, giving the campus a total operational building area of 230,000 m². A single central plant serves, and will continue to serve, all campus buildings with chilled water for space cooling and hot water for space heating and domestic hot water heating.

The Cal State Fullerton central plant generates high temperatures hot water (HTHW) for campus space heating and cooling needs. Three HTHW generators produce 177° C water at 41 kPa, of which the majority is used to drive single-stage absorption chillers that generate 7° C chilled water. Hot water is also distributed to campus where the temperature of the hot water is stepped down to 82° C in building heat exchangers. This hot water is then delivered to heat exchange coils to maintain 35° C air for space heating and 60° C domestic hot water.

Presently there are three 8.8 MW HTHW boilers, each capable of firing either natural gas or No.2 fuel oil. All three boilers are the watertube type with thermal efficiencies of 75-80% when firing natural gas.

The central plant has a total of three chillers consisting of one 2,638 kW and one 4,045 kW, HTHW, absorption chillers, and one 2,638 electrical centrifugal chiller in conjunction with a eutectic salt thermal energy storage (TES) system. The Cal State Fullerton central plant utilizes four cooling towers for condenser loop heat rejection. There are three 175 L/s cooling towers and one 265 L/s cooling tower utilizing respectively three 30 kW and one 60 kW cooling tower fans. The cooling towers were built in the early 1960's and are considered beyond their useful life.

When compared to modern DHC plants, the nearly 30 year old Cal State plant is relatively inefficient. In terms of achieving high energy efficiency, to create the 177° C HTHW is inherently wasteful, considering the ultimate use of the energy is to produce 35° C water for space heating and 60° C water for DHW. Also, there is no capability in the present system to reallocate available waste heat energy and in fact the chillers operate around the clock to get rid of this heat. With the existing plant beginning to exceed its useful life and with the campus poised for a major expansion, the University decided to upgrade the central plant into a more energy efficient and environmentally friendly facility.

Measures Taken to Achieve Environmental Benefits

Upgrading of the Cal State DHC system, which is currently underway, incorporates both energy efficient technologies and modern operational concepts to improve overall efficiency. These are summarized below.

The new distribution systems at Cal State will distribute hot water and chilled water to the campus, and will be variable flow distribution systems. Unlike a constant flow system, where the distribution system pumping capacity is selected for, and often operates continuously, at a rate corresponding to a peak hour condition, a variable flow system tracks the demand, delivering only enough flow to satisfy the short-term cooling and heating demands. The savings in pumping horsepower are substantial since horsepower is reduced as the cube of the reduction in flow. In addition, the existing 5.6° C delta T chilled water system is being converted to a high delta T chilled water distribution system, with a delta T of 13.3° C. This has the effect of reducing the flow by almost 60% since every litre of chilled water carries almost 150% more cooling energy.

The original relatively insufficient single-stage absorption chillers, arranged in parallel, are being replaced with more efficient electric motor driven centrifugal chillers arranged in series. The series arrangement is the most efficient since, unlike a parallel arrangement, only one of the three chillers must produce the cold 4.4° C chilled water output required of this system. The other two chillers operate at higher output temperatures and consume less energy.

The heat generated through normal activity in campus buildings, which would normally require rejection through chiller/cooling tower packages, is under the new system captured for space heating at other needy locations on campus. To accomplish this, the first chiller in the series line-up is a heat recovery chiller. This unit is used to extract the available heat from 18° C chilled water return. At the same time, this same unit, without increasing its electricity demand produces chilled water supply for space cooling and low temperature hot water supply (recovered from elevated condenser heat) for space heating and DHW heating.

Furthermore, all chillers at the plant will operate on R-134a refrigerant (an HFC) which has an ozone depletion potential of zero.

With time-of-use electrical rates, generating chilled water for space cooling in the middle of the day is an expensive proposition. Chilled water thermal energy storage (TES) was identified as a key central plant component to alleviate this concern. The centrepiece of this system is a 10,000 m³, above-ground, chilled water TES tank. This tank is sized with sufficient storage so that the electric driven chillers will be completely off-line during on-peak electrical rate periods. As a result, there will be almost no central plant electrical demand contribution to the campus peak electrical demand. Other benefits fall out of this TES strategy. With TES in place, chiller and cooling tower operation can be regulated to cooler periods of the day, thus their operation becomes more energy-efficient. Chiller operation also occurs during the higher heating demand periods of the day thereby maximizing the prospect of useful heat recovery.

In that space heating and domestic hot water heating are to be accomplished with recovered chiller condenser heat, creating a coincidence of chiller operation and the heating demand is important. Also, with only one heat recovery chiller, the ability to store heat for later use is important. Therefore, 1,140 m³ of hot water TES is also being implemented.

With both chilled water TES and hot water TES, the campus has an excellent level of flexibility in the use and reuse of its energy resources. This flexibility translates into cost-effectiveness and energy efficiency of the DHC system.

Summary of Environmental Benefits

The now under construction all electric central plant involves a drastic reduction in air pollution emissions - 97% NOx reduction over that of current emissions attributable to the Cal State Fullerton central plant. This is accomplished by removing the combustion processes from the local site and instead using electricity generated at the utility power plant where efficient, large scale, industrial grade Best Available Control Technology is used.

Several central plant development scenarios are presented below to illustrate the NOx emissions associated with each scenario. Scenario 1 represents the central plant as it was before any upgrading began. Scenarios 2, 3 and 4 represent possible alternative upgrades that could have been considered at different time frames. Finally, Scenario 5 represents the central plant now under construction at Cal State, Fullerton.

Scenarios

Note:

The source report for this case study does not quantitatively report the magnitude of energy savings expected due to the upgrades discussed.